Before an exam, I always go through as many past papers as possible but I still don't know how to do the questions on an exam when they come up, why and what can I do?

Despite getting lots of practice being very important, maths is not really a memory based subject and you shouldn't be just going through the all the questions before you have tried to fully understand the concepts behind it. During revision, it would be much more efficient to first check your understanding of the concepts by going through the textbook chapter by chapter and looking through worked examples for areas you struggle with then attempting to redo that question yourself. After you have cleared your head on all of the topics, then you can find some past papers to really test your understanding. It is not about how many papers you get through, but if you have really understood the concepts behind them or not- otherwise you might still not know how to do a similar question if they come up on an exam.