I know this isn't much related but, in GCSE, I was working at a Grade E at the end of Year 9 but got an A grade at the end of Year 11. This made me complacent I think.
At the moment, you may believe that learning maths means learning rules that you have to regurgitate in the exam. As you've discovered, this doesn't work. Think of it like this; if you see a cooker on with the gas/elec on full blast, do you think to yourself 'I must remember not to touch that'? Do you worry you'll forget? No, because you understand what would happen if you did. Same with learning maths (or anything). If you can understand it - you won't forget it.
To really understand it there are 2 good ways. One to be able to explain the idea without using the word itself. So for example, saying 'differentiation is when you differentiate' is meaningless. You have to explain why you do it and what is its purpose.
Second, can you explain it to someone else? If so, and they understand you - you understand it!
Feel free to keep asking me questions
Would it be possible for you to list what you need to get for a D, C, B and an A?
I did my AS maths exam and got an E. In class, I was sending my teachers past paper questions and getting him to mark, studying before the day of the class. I was working at Grades A and B throughout the year. I just don't understand where I went wrong. Can someone with advice tell me how to improve my grade at A2 please? Thanks
Just do all the past papers backwards, start with all the hard questions and go backwards, it worked for me. If you get stuck on the last question ask a teacher or friend
We have a textbook we use. Should I understand each topic and do questions/ past papers after before I move to the next topic?
Do as much maths as you can. Forget exam boards etc, I taught myself from a text book that was different from my actual exam and completed past papers from all sorts of exam boards.
There's a difference between knowing how to do things and understanding why you do them. Once you understand you should be able to do any question.