I think you have test anxiety, it's very common. If you can self test at an A level, anxiety is preventing you from receiving A's that are actually included in your records.
I had a professor years ago that suggested that when taking a timed test, avoid making mistakes by first going thru and answering all the questions/problems you are sure of. Don't spend anytime on something you're uncertain about. After you finish the items you are sure of, go back to the ones not completed. Repeat this process and and sometimes (quite often) the ones that gave you trouble, you will be more comfortable answering. Don't worry about being the last to finish, just use all the time to answer and then review. By that time you should have improved your chances of acting the test.
I think you probably are an A level, but it doesn't count unless you actually get the A. Once you can figure out how to relax you will do much better. Good luck.