Ok, first thing's first - stop stressing.
You've GOT to stop stressing. The likelihood of why you're doing so badly is probably because you're working too hard. My mum as a teenager at one point panicked about her exams so much so that she worked 7 days without sleep or something crazy like that. She came back with a fail. Normally in school, she's top of the class.
And to use another example, one of my good friends - she has to work a lot harder than most to get the same grades. And she worked so hard that she was really really stressed and she aimed for all the wrong grades (eg, unrealistic grades) and she came back with disastrous results at AS. At A2, she changed her tactic - she decided that she would work hard but she's not gonna let it make her stress. In the end, she got good grades that she could never have dreamt of getting a year ago, and got into her dream uni, University of Nottingham.
If you're not eating and sleeping properly, that's gonna affect performance - you're not gonna be able to think straight and absorb as much as you could've when you work.
You need to learn to relax - be it listen to music, write, read a book, go out for a walk, go out clubbing or hang out with friends. Go out! For every hour that you've worked, give yourself a 10min break. Give yourself treats, like if you'd got through this chapter today, you would treat yourself to a top you've always wanted, or a show you wanna watch on TV, or just go and make yourself a cup of tea. Reward yourself. If you'd been working all week, find one day out of the week where you will NOT work. Don't think about work, don't even touch any textbooks - but just do something you'd really enjoy and love. Arrange to meet a friend at the weekends so you have something to look forward to. Work completely without relaxation is destructive and it makes you do badly in exams.
And also - aim realistically. If your ability levels are B grades, then aim for an A but be happy with a B. If your ability levels are C grades, then aim for a B but be happy with a C. Know your abilities and know your level - you can't be good at everything or perfect at everything. If your level is a C grade and you're aiming for an A, you're simply setting yourself up for disappointment. But of course, likewise if your ability level is an A grade then aim for that A. At the end of the day, you must be realistic about your abilities.
All the best
