No need to worry - you still have plenty of time. Anyway I'm an A2 student now so have had public exams for the last 2 years so I'd say the following:
1. Be honest with yourself about where you are in relation to where you want to be. If you want an A* in maths say and get an A* with no revision currently, then you probably don't need to spend huge amounts of time on maths which could be better used elsewhere (obviously don't get complacent though), and obviously the opposite applies where your current achievement is drastically dissimilar to your aims.
2. If you prepare properly, you should never be worried about an exam. As long as you have prepared properly the exam is just an opportunity to show off what you know. At the upper ends of the spectrum, i.e. you want an A*, for stuff like sciences and maths you should be able to get to a point where you know everything they can ask you, and if you reach this point there is no need to be worried because they can't ask you anything you don't know (use the spec). If you get stuck in the exam on maths or sciences, you just need to work out what part of knowledge which you already have if you prepared properly can be applied to the situation in the question.
3. Revise hard but also make time to do other stuff. As a general rule I typically don't do revision after around 8:30 meaning i have plenty of time to relax watch tv etc. Obviously in the middle of exams you've got to do what you've got to do, but if you know you are ready for something don't feel as if you have to do more just because other people are.