I can't be much help, and I really hope stuff improves for you. But regarding the grades, have you checked what grade boundaries your teachers are using on tests? Especially with the sciences they're often a lot lower on the actual tests than what is used in class (i.e. sometimes as low as 60% for an A instead of 80%), I went through AS bio getting like Cs and Bs all year because my teacher marked like that, but got an A in the exam with the same knowledge. Don't count on it, but it's just something you might be able to check? Also, chemistry is ****ing hard! So good luck!
Re. managing time, remember to prioritise the stuff that will actually go towards your AS (if you're doing AS that is), so any art coursework over a chem in class test. Also, make sure you have some breaks - i know, not exactly groundbreaking, but you'd be amazed what a ten minute dog walk can do for productivity etc - and, if you're not doing actual AS exams remember that you're working to a two year deadline not that little test week. Don't care about every little thing that doesn't count, especially if it's going to screw with your mental health.
Best of luck