I wish I could give you some great advice but I can't. I try to remeber to think about the positive. I figure up an estimate of how much of the information on the test that I know and asked myself am I going to get much more into my head. If not I go take the test and let myself fail or succeed. I suppose you could also remind yourself that you can take the class over if you need to. If you do fail, what is the worse that can happen? I suspect that you could live through that.
I tend to give up too due to perfectionistic/negative thinking. I hope you figure out a way to curb your perfectionistic thinking.