This honestly sounds like a case of frustration building up to a case of nerves. I'm not a psychologist, just getting my Dr. Phil on, calm down, relax, take a deep breath, etc. All that negative energy serves you no good, it is in fact causing the opposite effect of what you are trying to acheive.
Perhaps you are subconsciously telling yourself that you don't know it, that your not good enough, and the first failed test confirms it for you. Again, let those thoughts, if you have them, go. They are not true, and they do not serve you well.
Take a breather if you need, not too long, just a couple of weeks, try not too think about it, remind yourself of all that makes your life good and worth living. Do all the things that you like to do, hang out with the people that fill you with a sense of light-heartedness and purpose.
Pick the book up again, re-read. Figure out what enviroment you thrive in, Alone in the morning with tea or coffee on the veranda, in the afternoon at a library with other students discussing hematomas, etc.
Then, when the test comes, remind yourself that the fear/anxiety is an illusion, a false belief. Also, caffiene can increase feelings of anxiety, so if you drink it, cut back a bit, if not, do something that relieves your stress.
Also, you might see about beta blockers. This is really a last resort, but if you honestly have problems with anxiety talk to your doctor or therapist, and they may decide that a prescription might work for you.