Stop trying!
I know that sounds drastic, but ask yourself why you work hard and get those fantastic grades? Is it because YOU want them, either for their own sake or to pursue a particular career that YOU want to do? Or is it because your parents/teachers will be proud of you if you do?
So, let's assume you don't want to be a doctor/lawyer or anything else that warrants the best university education you can get. Could you get by with a B? That's still a good grade, and even C wouldn't be the end of the world, would it?
Then think about what you really want out of life. Not breaking your back to please everyone will give you a little time out to contemplate this. Go out with your mates, try a few different activities - anything that will put a little perspective on life.
True success is inner happiness.