Same as me. I got A*'s in Chemistry and Physics, and A's in Maths and Biology, so maybe I can help here.
The key is not getting too stressed about everything-when you are stressed it becomes really hard to focus. I got several good bits of advice on how to do this, whilst i was doing it AND since i've finished that, and my Bachelors degree. Many of these may be from TED talks.
1) Don't focus on the goal-your A's and A*'s. If you do, everytime you miss the mark you will become very disheartened, add to stress etc. Yet there is so much you can't control so you shouldnt be extremely stressed about it. Focus on what you CAN control-like the timetable for studying. You can control (largely) how much time you put in to study, how focused you are when you study, what you study etc. Focus on that and if you do it wisely, you will become much closer to being as best prepared for exams as possible.
2) Work with others to study. I found in most of my classes, I thankfully understood most of the stuff we studied-when i didnt understand something, someone else did. It helps to talk with those people and try and understand it from a different perspective.
3) Put in the work. I suffered in my degree because, unlike for my A-levels, I started slacking and spent a week or two every now and then where I barely studied. Commit to doing, say, a few hours each week for each subject.
4) Focus on your weaknesses. I went to a lecture by some guy who promised he had the key to getting an A* in my Alevels. And that was focusing almost entirely on the stuff you don't know. His logic was if you decide to study everything, you risk getting overwhelmed. Why waste time on organic chemistry when its the compound structure stuff you struggle on? So check your practice tests, see where you went wrong, and focus more on that.
5) Stay focused. Don't do what i did in my final year of my degree-delude yourself into thinking you worked hard. The best way to stay focused is to work for short bursts-30 mins to an hour-and have some kind of incentive at the end of that time (cake/food works good). That way the part of your mind that wants to procrastinate can be suppressed long enough to get work done.
6) Ask lots of questions. I work as a private tutor for A-level students, and while I dont want to advertise myself and I know it wont work for everyone-your teacher is there, and they are free. If you are in private school-even more reason to get your moneys worth. Ask them questions, ask on here, work out those uncertainties.
7) Learn the style of questions. Actually do practice exams-do all of them. See what kinds of questions you are going to be asked. See the mark scheme model answers. That way you begin to start understanding how you get the marks. I found when I did this, when i went into exams, I knew exactly what they wanted me to say for so many questions.
Hope this helps