I used to get really anxious about getting perfect grades all the time, too. I still do, but not as badly. The way I helped myself was every time I stressed about a grade, I asked "what's the worst that can happen?" And I would have a conversation with myself:
"What's the worst that can happen?"
"I fail this test."
"So then what happens?"
"My grade in the class goes down."
"And?"
"If I don't do well, I'll have to retake the class."
"Well that would suck, but could you survive it? Would you learn from it?"
"Yeah."
"Ok, not so bad then."