ok. Go on a run/cycle. Or join a sports team if you hate running and cycling. Exercise helps take your mind of things and makes you feel good about yourself and increases productivity. Or atleast that's what I find true for me.
Secondly, remember that exams aren't there to trick you and ruin your life. they are there to tell you your ability. Imagine going to the opticians and getting an eye test. The eye test tells you whether you are long or short sighted, whether you need bi-forcals, varifocals, normal glasses or no glasses. In the same way a GCSE or A level tells you how much you know about a certain topic. They tell you whether you are cambridge material, whether you are fit to be in a Russle group uni, whether you would survive on a medicine course or whether you should just be doing the average degree at Nottigham Trent (which there is nothing wrong with!) You need to stop worrying about what will turn up and just treat it like an eye exam. You will likely get the grade which reflects how much you have learnt in the last two years- providing you do not stress too much, which is the biggest cause of underachievement in exams.