what helps me is when you do practise tests, identify which questions you got wrong, figure out what the actual answer is and how to get that and make a note of what topic its in (e.g. if it was a trig question) and then at the end of marking you can see the main topics that you didnt understand. then i revise those topics (either by watching youtube vids - i like gsce maths tutor, or by using my notes or textbook. then like the people above said, drfrostmaths is really good (i love drfrostmaths its honestly the best maths revision source ive used so far) or the teaching it to someone else helps as well though it can be the wall or a pillow, not necessarily a friend (i find that this helps me to understand it, though not always how to apply it to questions)