You're in the exact situation I was in about January (when I got my Dec mock results back). I went from a C in December to 2 marks off an A in my March mocks. If you can prioritise Maths do it so you can spend as much time doing valuable maths revision as possible. However don't ignore other subjects if they're important (for example you want to do that subject at A level). For revision I completed past papers, marked them and revised where I lost marks. If you can get your mock papers back, do it because this (and doing past papers) will allow you to know where you went wrong (obviously) but should also help you to develop a better understanding of maths as it's not an easy subject for many. However spending 2 to 4 hours at a time revising maths is a waste of time, revise effectively by taking breaks to relieve stress and allow what you've learnt to sink in. Take whatever advice or revision sessions about maths as you can including asking your teacher when you don't understand something and going to after school sessions. I'm not going to lie but it will be difficult (unless you're good at maths) because it's quite close to the exams, but if you are really determined, you should get that A/A*. Good luck!