If you're doing a question and don't know how to do it, don't just immediately open the mark scheme and go from there. Finish the question to the best of your ability, move on to another question or two. Then go back and retry the question. Actually put in effort as if you're in an exam. Then check the mark scheme and try to actually understand how to do that method, don't just memorise it, it won't work. Then like a week later come back and do the SAME question again, and ensure you know it. If not, go back AGAIN the next week after checking the mark scheme and do the SAME question (don't just memorise it, try to really do it). Do that for every question and make sure you really get it.
Something I found useful was don't stretch yourself too thin. Don't do ALL of maths, pick a unit, master it, and come back to it a month later and do it again. Over time, you'll get really good at some topics, so cover them once every 2 months or something.
Something I'm guilty of is if I forget a question, I peek at the mark scheme, think "Oooohhh of course" and then trick myself into thinking I know how to do it. ^THIS^ is the worst possible thing you can do, trust me 😭
Most importantly, remember, even if it's slow progress and you keep getting things wrong, don't give up! It's possible to improve, YOU'VE GOT THIS!
Edit: I just saw that this is 6 months old, oops. How's it going?