Remember adrenalin, the "fight or flight" hormone? Now, think in terms of evolution: We have progressed as a civilization far faster than we could keep up with, as a species. Our physiology still goes on alert for imminent danger close by. When the brain connects just the right neurons to set off that danger signal, adrenalin is released, floods our systems, and gives our brains two choices with no time to think, only to react: Beat the lights out of the adversary or run away from the danger.
Of course, neither option applies to taking standardized exams. Who are you going to beat up? No one! Where can you run? No where! "OMG", says the neuro-endocrine system, "More adrenalin! The situation's worse than we thought!!!!"
<Pause. Take a deep breath. Go do something else, for a minute, something soothing or mechanical, like washing dishes, playing Sudoku or solitaire, and then come back.>
Just as there are quick-trip switches in the brain to signal alarm, there are other reflexes to signal, "All is well. Relax and enjoy the break from dealing with saber-tooth tigers."
One, which is good for studying, because it takes a minute or three, is to stare as far off into the distance as you can. Evolutionarily speaking, in cave-woman or cave-man mode, if you were able to stare up at the stars, look out across the horizon, enjoy clouds or mountains far away, it physiologically signals that there is no danger, nearby, or else, you wouldn't be looking off into the distance. Eyes triangulate to measure distance for the brain. So, if you are up against a wall, even, and you let your gaze seem to stare through the wall and off into infinity, it has the same result.
Now, remember, that's good for studying and, if you allow time, relaxing in your car before stepping into the testing center.
Another, faster reflex is humor. Cave men and women didn't stand and laugh at saber-tooth tigers, you know. If you can break into a real laugh, from a sudden gust of humor, your tension can snap in a second, leaving your brain to function just fine.
Adrenalin, by the way, is short-lived. It seems to last so long, because it keeps being pumped out to replace what is rapidly broken down and turned over.
Anyway, things that make you suddenly, unexpectedly break out into a laugh are wonderful. Anything that makes you laugh is good. Real laughter interrupts the flow of adrenalin.
What I did for the first case of exam-PTSD whom I tutored was to gain his trust, then recognize the issue, then invent the funniest "yoga" posture I could. He followed me through a couple quick neck roll and stretching things and then, ever so slowly, into the most ridiculous position I could think of. I'd only told him it was a yoga exercise for stressed students, but that it works...
... like a charm! So well, in fact, that after proving it on his next test, he did it in front of all his classmates, at the final exam! They broke into laughter, making him laugh even more, and he did really, really well on the exam. The other students probably did better for it, too.
Cut back on caffeine, as it interferes with the enzyme responsible for breaking down adrenalin. Try these two tricks, if you can, and then tell me what you think.