Past papers are beautiful because 1. Questions are often repeated and 2. You can learn exam technique and key words that the examiner is looking for, so do past papers, under timed conditions and when you're done, mark them strictly. Go back and look at which questions you got wrong and go back and refresh on that topic aswell as learn the mark scheme answers for it. Repeat this past paper trick till you find yourself getting better and better and starting to hit A* grades.
Once you're done with all this you can relax a bit. Near exam time, don't panic as this can lead to forgetfulness and make sure you're well prepared, and go on and rock your exam Good luck! x