Try not to dwell on it... I know it's incredibly hard! If I do badly in an exam, I try not to rehash it in my mind, or discuss it heavily with others after the exam. Tick it off, and focus on the next one... what's done is done.
And if you have to think about it, focus on the things that you did well in the exam, instead of the mistakes that you made. Chances are, you are exaggerating the mistakes in your mind. I felt so bad after my French oral because i accidentally slipped in an English "and" mid way through... but now I'm trying to think more about the fact that I made barely any mistakes apart from that when it happens to cross my mind.
Keep the faith, keep revising (keeps yourself busy, distracted, and boosts your grade :P), and keep optimistic... everything will be just fine. And GCSE's aren't the be all and end all!