I do A level Biology, Chemistry and Maths. I really felt the same coming out of exams. I revised so much throughout these two years, and yet i felt i did not do as well as i should've. The truth is, you usually don't do as bad as you think you have done. The main thing to do now is to focus on your upcoming exams since dwelling on those past exams won't help. I know it is difficult and that you keep thinking about all the possible mistakes you have done, but try to leave those thoughts BEHIND you! and remember always, there are many many many possibilities if anything goes terribly wrong!
Hope that helps.