Sadly, the nature of GCSEs means that, a lot of the time, we will feel like we've do e terribly even if they haven't. I was convinced I'd failed science, but ended up achieving higher than my predicted grades. All you can really do after exams is to congratulate yourself for having tried as hard as you possibly could have done on it, and then try to forget about it. Most sixthforms have some level of leeway in their grades, so if you don't get the grades that you want you should still talk to admissions and se whether you could still get a place.