I agree with WhiteRav3n that peppermint is very helpful ,another boosters I use are basil ,rosemary and citrus fruits (lemon ,orange ,bergamot orange,tangerine,lime and any other mixed hybrids of those).I will rather spend some time visualizing me taking the exam with perfect score ,than really stressing and freaking out ,while thinking of how miserably I fail.
Another way is to practice in front of mirror and really get into the role of totally confident student that gets all the exams.
Also, I often spend some time visualizing things, before I fell asleep the night previous of the exam.
I guess we are as big as our dreams are.Why putting yourself down ,when you can work hard and see yourself as a winner?
I know it is hard to get rid of such mentality ,but who has desire finds ways.
Breath ,smile ,study hard and go take the exam knowing that you did your absolute best.