How do you deal with stress over a potential bad grade?

I took my exam yesterday for a post grad class in psychology, and i feel as though it went catastrophically wrong. I studied my arse off for it, but i panicked when i saw the questions and feel as though the answers i gave were well below the standards i should have given (and below what i normally would give). My main concerns are that this class is a part of series of classes that are there to determine whether i can advance on to a Masters degree. I need a distinction average, but i feel id be lucky to get a pass after yesterday.
I suppose i just want to know how can i get rid of these feelings im having. I dont like feeling like this. I want to feel the same safety and security i had before i went into the exam. I dont want to worry and stress out. Is there anything anyone can suggest will help bring me back up again?