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Two things:
1) Figure out constructively what caused you to score less well than you wanted, and come up with concrete ways to fix it. For example, there was that suggestion of skipping harder questions so that you can come back to them. You've had your venting/complaining session, now fix the problem.
2) Please try to relax. I got a couple B's in chem, OMG WAY WORSE THAN A-. Heh. I know it is hard from your perspective because of where you're at in the process, but you have a long road ahead of you, and this one this is NOT worth stressing over. If you had gotten a C... well then I'd be worried (but even then, you wouldn't be screwed). There are more important things to worry about.