Mistakes can be uncomfortable, truly. I used to obsess over them. They still get me sometimes, but I've become much better at handling them than before.

What helps me is (1) asking, "How much will this really effect me in the long-run?" For example, your school assignment might receive a poor mark, but how badly will it effect your class grade? How about your overall GPA? (2) Asking myself how I would perceive someone else making the same mistake. Perhaps not overly helpful for your current situation, but this often calms me down when I make a mistake in front of others. And (3), remembering that mistakes are necessary for learning and success. Both learning and success eventually require pushing yourself and taking risks, which inevitably will lead to some mistakes.

I also like Nike007's point, that mistakes are in the past and worrying will not change them. So why dwell on them?

Best of luck to you.