Oh okay. Just worried because I'm usually rubbish at exams. Not that I don't understand anything or remember anything, just pressure etc makes me do bad...hopefully it'll be okay!
As Franklin Roosevelt said, the only thing you have to fear is fear itself.
As a former teacher, I am well aware of the type of test anxiety you describe. There are many people for whom tests are not an accurate measure of their achievement because of this anxiety. I wish I could give you a sure fire way to overcome it, but maybe what follows will help.
One thing to do is to keep telling yourself that you know the stuff and take deep cleansing breaths when you feel anxious. Believe it or not, the way you breathe has a lot to do with it. The shallow breathing associated with fear causes that fear to spiral. Your fear has a physical source: carbon dioxide buildup. When carbon dixide builds up, you get a feeling of panic, and that panic can build as you continue poor breathing. If you feel anxiety, try something called diaphragmatic breathing. Inhale slowly and deeply, using your stomach to initiate the breathing. Then exhale slowly and very deeply. Get rid of all that bad air! Do that a couple of times and you should feel better.
With all PADI students, after the test is completed, the instructor goes over every question you missed to make sure you understand why you missed it. When I teach the academic portion of a PADI class, I constantly monitor student learning, and I don't start the final exam until I am sure everyone has things nailed. All my students get scores in the high 90's except for those with test anxieties such as you describe. Thye generally score in the 80's, whch is still comfortably passing. I have never had anyone score lower than that.
When I go over their answers, there is a lot of head slapping--"I knew that! Why did I put that answer down?" No problem. Going over those answers individually ensures that your test anxiety is not the determining factor in getting through the class.
Relax and let the process work for you.