Hi,
Above all just remember, there are many other people out there who are in the same boat as you, including myself. People have different ways of handling stress, I personally find talking to other people helps, go and sit in the garden or just do something (like reading) that takes your mind completely off exams and what you are supposed to be studying.
At the end of the day getting yourself so worked up isn't worth it - try not to see the exam as if your future depends on them, so you might not get all A's and B's, but it isn't the end of the world. If you have worked hard this year, and have put in a decent amount of revision, nobody can expect more of you than that - so don't expect if of yourself (I know it's hard - I can work for 6 hours and feel like I have done nothing).
Exams are important but they really are not everything, there are many people in much much worse situations, and whenever I feel sorry for myself I try to think of them and get things into perspective (I am not suggest you make yourself feel guilty!!).
If you are a recovering self-harmer then you obviously know your limits. Stick to them. If you don't feel you can manage any more revision or you feel nothing is going in, and that pushing yourself further simply makes you more stressed - stop. I am not a biologist but I do know that our brains can only take so much and the consequences can be dire if we push them too far.
Have a break tonight, do something different (maybe just watch TV), or have a bath perhaps. Get an early night. Get up in the morning with your stuff ready, have a good breakfast - go in the exam and try your best.
Good Luck xx