Haha, I know the feeling. When I got my A level results and went to talk to people they were like "Oh, did it not go well" when they saw me but I got good grades.
If you've just finished GCSEs then you really have nothing to be stressing about. Literally, you don't even have any work you could be doing let alone should. I found I used to get really stressed, I'd angry about the most trivial things and could never relax until i started exercising so maybe try that? I think having a physical outlet is a very good thing and what i was missing. You could take up a sport if just exercise is boring or you enjoy competition.