Do this:
Read a few pages from ur notes and then take a few minutes break. Go outside or browse net. After a few minutes u'll feel that I should prepare for exams instead of these time wasting things.
Then again start ur preparation.
Just relax buddy. Everyone has to face this situation. These r just exams nothing else. So just calm down and take a long breath and start study.
I know u'll do a gr8 job and will get good marks.