I did my GCSEs at a college, due to being home educated. Although I was 15 at the time, the course was mostly populated by people who were re-taking them for one reason or another. One of the girls there was 22 - she'd done badly at school, but wanted to be a lawyer so was re-taking her GCSEs so she could get on with the rest of the education she'd need. She was ten years older than you are now, yet still very young and setting off on a path that was making her happy. So don't feel that you have to get everything done now, this instant, or even as soon as the average person - the main thing to do is to keep yourself confident and happy and to have faith in yourself to follow the direction that appeals to you. Eventually you'll get what you want, and as Claradoon says, at the very least you'll be able to be happy in the meantime.
Another thing about the grades - there are increasing numbers of vocational courses you can do, so even if you're not academically inclined you can still get a good job. A lot of people start out in apprenticeships and end up in management positions in Admin, IT, Engineering, Construction, Carpentry, Plumbing, Electrics, Zookeeping ... well ok the last one is slightly obscure, but hopefully you get what I mean. And even if you're set on university, if your grades aren't as good as you hoped, there's nothing wrong with re-takes (I'm having to re-take two A-Level exams, along with a lot of other students). And then there's the thing where universities have placements left towards the start of term, so lower the requirements .... essentially, you'll do fine. Try not to stress.
Hopefully that wasn't too long and rambly ...