Why do I make mistakes when I'm over excited or over stressed?

How do I stop making mistakes when I'm excited or stressed? Does it have something to do with my concentration? How do I cure this?
(PS:I knew the answers to all the questions to a quiz and I made mistakes, some because I didn't recheck, and some because I was stressed about the time limit.)
Why do I make mistakes when I'm over excited or over stressed?
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