You most definitely can do it, for chemistry it's a matter of knowing the markscheme rather that becoming complacent on learning and relearning the text book. My advice for chem in particular would be understad the topics then get your hands on past papers from 1990 minimum and work through them towards the new ones, I know the spec has changed since then but a lot hasn't so just miss the ones not in your spec, by the time you get to last year's paper you'll be scoring 100% as questions just keep repeating year upon year. For bio it's pure textbook and then knowing how to approach the data questions. Maths, I initially got a low grade in my December mocks yet went on and achieved near 300 ums in the real thing (again a matter of past papers and thinking outside the box). Just believe in yourself and keep working, make sure you balance your life as it will drastically improve the quality of study achieved when you study. Best of luck
P.s don't listen to any of these people putting you down! If people can't achieve something they'll try make you think you can't either! Aim for the sky and you'll get half way, best of luck.