I'd have a look at what you're eating as well as the exercise that you're doing. You may be eating too many calories but if you're doing a lot of exercise, it's quite possible you're not eating enough too. You should be aiming for 1600 or thereabouts a day, after exercise. So if you burn 500 calories, you can eat 2100. You need to be careful of going much lower than that because you'll end up losing a lot of muscle as well as fat which obviously isn't good and it'll lower your metabolism and so you'll find it harder and slower to lose weight.
I'd suggest using myfitnesspal.com to track everything that you eat and all your exercise. I find that on the days I don't count everything and just guess, it easily adds up to a lot more than I thought if I do sit and work it all out.
My biggest piece of advice that I'd have for anyone is to plan. Plan out what you're going to eat that day and what exercise you'll do. I know if I do that I stick to it a lot better than if I just go to the fridge, freezer, supermarket etc. and go hmmm what to have. That way you know you're within calories and you know what you've got left over for snacks if you're hungry at any point between meals.
I don't know what happens with the snacking on crisps, chocolate etc. but I know that if it's in the house, I end up eating it sometimes. If it's not then I generally don't even miss it. If you're 17 I'm guessing you probably still live with your parents, maybe speak to them and get them not to buy in those kind of things?