I agree. Chemistry is such a difficult A-level, and you don't want to pick a subject that you find hard. However, just because you find it hard at gcse, doesn't mean you'll find it hard at a-level necessarily. Vice-versa happened to me. Perhaps look into the a level specification and see if its something that you would find interesting/would be able to manage. If not, then perhaps find another course that doesn't require a level chem.