First Relax. It's a long road to a 4.0 so slow down and take it easy. If you put all this pressure on every quiz, test, and assignment, you'll never make it because of your self imposed stress. Just remember to try your best and good things will come. There's plenty of time to erase one quiz. I imagine there are three or four exams plus a final that have much more weight than one quiz. Also if you're having trouble with chemistry you need to do more problems. From Gen Chem 1 to Organic 2, if you do the majority of the problems at the end of each chapter, you will get an A. Also the easiest way to lessen the stress is to study a little every day. I know that's easier said than done, but if you cram you won't make it. And if you're still having trouble, meet with your professor and get his advice on how to get a good grade. You'll need to get to know a few professors anyway for you LOR, so why not start now. Good Luck
and remember...you've only failed if you didn't give it your best. If your best doesn't get you into dental school than you still have alot to be proud of. Now get off SDN and get back to studying.