If stress is causing major problems for you, then good preperation is probably the answer. If you go in knowing that you are prepared for each of the question types, you are less likely to be stressed by what you will face.
Dealing with stress is a major issue for an exam like this. If you are stressed your mind will wander and you cannot concentrate on even those problems that you are very capable of answering. I can also cause you to be tired and/or sleep poorly.
I would suggest taking some time and getting an honest assessment of your own skills. Then, set out a plan to tackle each of the areas in which you need to be stronger. If you can point out your preperation and test experience, others here can probably help with suggestions.