Don't worry about it, kid. I'm assuming, if you've just got your GCSE results, that you're 16. There's nothing you can screw up at that age that you can't come back from. Believe me! I got 6 GCSE's, failed 6th form at 17, then just did a lot of nothing for 6 or 7 years before entering 2 years fairly heavy drug abuse when I was 23/24. A massive mental and emotional collapse followed, after which I became a recluse for about 5 years, completely missing my mid-late 20's, barely working, barely going out, barely talking to anyone. I did, however, manage to get myself out of the house once a week to do 'A' levels in English and Psychology, completed my degree last year and am now in the process of applying for my masters.
It's taken me around 10 years to get over all the shenanigans, but the point is that at 16 you should not be worrying about your life being ruined. It isn't. It's just beginning.
The future is unwritten, to quote the great Joe Strummer.