This accumulated pain is a negative energy field that occupies your body and mind. If you look on it as an invisible entity in its own right, you are getting quite close to the truth. It's the emotional pain body. It has two modes of being: dormant and active.

.The pain body wants to survive, just like every other entity in existence, and it can only survive if it gets you to unconsciously identify with it. It can then rise up, take you over, "become you," and live through you. It needs to get its "food" through you. It will feed on any experience that resonates with its own kind of energy, anything that creates further pain in whatever form: anger, destructiveness, hatred, grief, emotional drama, violence, and even illness.

So the pain body, when it has taken you over, will create a situation in your life that reflects back its own energy frequency for it to feed on. Pain can only feed on pain. Pain cannot feed on joy. It finds it quite indigestible.

Once the pain body has taken you over, you want more pain. You become a victim or a perpetrator. You want to inflict pain, or you want to suffer pain, or both. There isn't really much difference between the two. You are not conscious of this, of course, and will vehemently claim that you do not want pain. But look closely and you will that your thinking and behavior are designed to keep the pain going, for yourself and others.

If you were truly conscious of it, the pattern would dissolve, for to want more pain is insanity, and nobody is consciously insane.
The pain body, which is the dark shadow cast by the ego, is actually afraid of the light of your consciousness. It is afraid of being found out. Its survival depends on your unconscious identification with it, as well as on your unconscious fear of facing the the pain that lives in you. But if you don't face it, if you don't bring the light of your consciousness into the pain, you will be forced to relive it again and again. The pain body may seem to you like a dangerous monster that you cannot bear to look at, but I assure you that it is an insubstantial phantom that cannot prevail against the power of your presence..

So the pain body doesn't want you to observe it directly and see it for what it is. The moment you observe, feel its energy field within you, and take your attention into it, the identification is broken. A higher dimension of consciousness has come in. I call it presence.

You are now the witness or the watcher of the pain body. This means that it cannot use you anymore by pretending to be you, and it can no longer replenish itself through you. You have found your own inner strength. You have accessed the power of Now.
Unconsciousness creates it; consciousness transmutes it into itself.

The pin body consists of trapped life-energy that has split off from your total energy field and has temporarily become autonomous through the unnatural process of mind identification. It has turned in on itself and become anti-life, like an animal trying to devour its own tail. Why do you think our civilization has become so life-destructive?
But even the life-destructive forces are still life-energy.

Let me summarize the process. Focus attention on the feeling inside you. Know that it is the pain body. Accept that it is there. Don't think about it - don't let the feeling turn into thinking. Don't judge or analyze. Don't make an identity for yourself out of it. Stay present, and continue to be the observer of what is happening inside you. Become aware not only of the emotional pain but also of "the one who observes," the silent watcher.

This is the power of the Now, the power of your own conscious presence. Then see what happens.

For many women, the pain body awakens particularly at the time preceding the menstrual flow....If you are able to stay alert and present at that time and watch whatever you feel within, rather then be taken over by it, it affords an opportunity for the most powerful spiritual practice, and a rapid transmutation of all past pain becomes possible.

From: Companion Tape to The Power of Now

The following is a transcript of a section from Eckhart Tolle's tape, Companion to The Power of Now. It needs to be heard, but for those that don't have the tape, a second hand transcript is better then none. Eckhart is talking about the pain body - a kind of energetic entity that takes possession of the human body, and uses it to generate more pain.
Eckhart Tolle:

It goes deeper then that and we are not running away from the fact that in you there lives a field of residue of past Human pain.
On a personal level, pain from childhood that leaves residues, Energetic residues….different things painful things have happened to you in your childhood and of course far beyond that in time. But lets just talk from the level of your present existence in this form.
The pain that is carried over from your childhood and beyond and leaves residues …doesn't go away completely, it leaves residues of pain, Energy fields, everything is Energy. And then those Energy Fields get together, because they feel, they vibrate at the same frequency. So emotional pain gathers into one mass of pain, contracted Energy, that is not flowing freely. An energy that I call the “Pain Body”, that humans carry inside them.

An energy field of pain.

And if you don’t recognize that, no matter how much Spiritual Practice you do and even what we are doing here if you do not see, if you do not recognize how the Pain Body operates, it will trick every time and you will lose consciousness through it.
First we realize “there is an Energy Field of pain in me”, that may manifest either as turbulence, anger, heaviness, depression, tightness, fear. Whatever way it manifests its not pleasant its painful its past pain but very much alive still.

You carry it inside. But you are not always conscious of it and I talk about it briefly in one Chapter of the book, so we go a little bit more deeply now, and now that the added Energy of Presence that we take into the Pain Body.

The Pain Body is sometimes dormant and sometimes active. It becomes active when you suddenly feel it very strongly, when it needs to feed on further experience of pain. As it always has to do regularly like, it’s a little entity almost, it needs Feeding, temporarily on the experience of further pain, and you will know this has happened when a relatively insignificant trigger produces an Incredible out burst, reaction of Pain. A minor thing goes wrong or somebody say something, or your partner says something or does something and there is suddenly a deep or even a thought come in, and suddenly it serves as a trigger for an immense, the arising of deep emotional pain.

At that moment the Pain Body this Energy Field has come up out of its dormant state, it is ready to feed on more Pain.
I haven’t put it that clearly in the First Book I will explain it in the next boo (Laughter)

There are two ways in which it feeds, because that is all it can feed on .Pain Body consists of that energy of pain its needs (ET says) “please where can I get more Pain”.

It feed on your thoughts, when it’s ready to rise up, it will control your thinking, it rises into the mind and your mind, which is your thought activity, because with the Pain Body every thought you think is destructive and painful.

And the Pain Body loves it!!! It gobbles up the Energy of every thought that you think every destructive and deeply negative thought, it eats it up, so to speak, (ET makes chomping sounds while all laugh) And it’s having a Good Time! It’s feeding and, at that moment, one way of putting it is, one could say “the addictive quality of Human Pain". Addictive because it loves it’s pain.
When the Pain Body has taken you over and has succeeded in pretending that that’s 'Who you are', all your thinking is completely aligned with it. And it’s feeding on it. At that very moment the last thing you want is to be free of pain. At that moment pain IS what you want because at that moment, You are the Pain Body.

And if I came into your life at that moment and gave you this message, “that Life free from Pain IS possible” You would hit me over the Head!!! (Laughter). Because the Pain Body would be there and I would be talking to the Pain Body and the Pain Body would be talking back at me.

So the pain body needs to come in periodically and we all have experienced it with partners now the pain body will use you thinking, it will feed on you negative and destructive thinking.

How long the feeding time lasts of any particular pain body varies greatly from person to person, I could be a brief one, it could be an hour or two; some pain bodies have a feeding time of several weeks, even months. That is the extreme and in very extreme cases there are some pain bodies that have virtually no dormant stage that are continuously feeding and active.

But that’s more rare but you sometimes meet people and the pain body is looking at you through their eyes and they are waiting for an excuse to have more. Tthey want you to give them a painful reaction. They want you to be angry with them; they want you to attack them. That’s people totally possessed by the Pain Body.

So Level One: the pain body feed on your thinking.
Level Two: The pain body feeds on the feed back of the emotional pain from other people

So it might not only use your own thinking it might even predominately use somebody else’s reactions. So the Pain Body feeds on thinking. The Pain Body feeding on others reactions. Now I mentioned last night the “unhappy me” a mind pattern that people are identified with an unhappy sense of self. This mind pattern, when the pain is active it becomes amplified, the Energy of the Unhappy me when the Pain Body moves into that mind pattern that already is telling you “your life has not been good enough” and so on and you may not make it, “life has passed me by”, and so on.

Now the Pain Body moves into that mind pattern and its Energy gets amplified, 10 times, 20 times, 50 times, 100 times in other words the “unhappy me”; the Pain Body arises, it flows into that mind structure, the emotion flows into that mind structure, the Unhappy sense of me becomes dreadfully Unhappy and Loves it’s Unhappiness because that’s what it consists of.

Now it is not a foreign body, which lives in you. There is nothing that is not Life Energy.

Even your pain body is life Energy except it is not flowing freely. It’s got stuck somewhere.
And when Energy gets stuck and cannot low freely pain arise like a river. If a river cannot flow the water accumulates and pressure accumulates. But really it's beautiful Life Energy.

The water is still beautiful there is pressure accumulating, inside. That’s what the pain body is.

Now, as we bright Presence, this consciousness in to the Pain Body, it can no longer fool you into completely identifying with it.

The presence, as the pain arises, from now on, and many of you are already practicing that. The Pain body comes up and at that moment you will recognize it as the Pain Body. That is the Beginning of Freedom from it. The recognition, when it comes, slight trigger, provocation, even a thought, deep Pain arises, in whatever form, in some people it is a very active pain, active aggressive pain, in other persons it is a Passive form of Pain, “poor little me” victim pain, does not matter which, pain bodies have different qualities in different people.

It maybe turbulence or tightness or constriction, it does not matter what it is. As it arises you will know there is a witnessing presence,.and you watch..

Where iss the pain body and where do you feel the energy field of it?

In your body.

You can feel it in the solar plexus or stomach area, as a dreadful sense of heaviness.

Some people perceive it as a big hole, a gapping hole inside.

Or intense anger.

But whatever it is you watch it, this is why it is so helpful, you sitting here witnessing, is witnessing Presence which is part of this new state of consciousness. It gets so much strengthened by sitting in this Energy Field.

So the Watcher will be there watching the pain so the pain can not use your mind anymore it can not feed on your thinking because you are watching it directly it cannot creep into your mind and then become an “unhappy me”.

All that’s left of the pain is an energy field of turbulence, or a heavy feeling inside you.

But the pain has not become an “unhappy me” a sense of self, of who you are. That’s the beginning of the end of the pain. But our practice is allowing “what is” to be. And that also applies as long as the Pain Body it there in you allowing the Pain that is there in you Also to be. That is a very powerful practice.

So I am not saying, we are not fighting the pain we are not even trying to get rid of it. You bring the consciousness, which is a very compassionate state of allowing “what is”. It is a very powerful state it implies you are Present. As the one who is able to allow.
You are the one who is able to allow the pain to be. You know you are not creating this pain at this moment because you are practicing accepting “what is”. So it is not pain you are creating, it is old pain.

You bring the “acceptance” to the old pain.