In terms of eating disorders, you need professional advice from a doctor or maybe a charity that deals with them issues. I don't think many people on here will be as qualified to give such advice.
In terms of the exams, you just have to change your thinking about them. At the end of the day, it is just questions on a piece of paper covering what you have learnt during the year. As long as you prepare for them, you should have no need to worry whatsoever.
I think everyone gets nervous about exams, I have felt nervous before exams even when I have been totally prepared for them; it's just natural I guess. You just have to utilise that adrenalin and use it to do well on the exams. It's not life or death, if you fail, there is the opporunity to do resits or maybe resit the whole year in extreme circumstances.
Everyone is in the same boat so just take comfort in that.