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A level Maths - Difference between understanding and remembering??

Ok, so I'm sure a lot of you have come across my constant struggle with A-level Maths, but here's another absurd question...

How does one fully understand the content throughout the subject rather than merely just remembering processes/content etc? I'm on the Edexcel spec but I find that the endorsed textbook only guides you through processes without really explaining why things are used etc. As a self-studier, this leaves a lot of gaps in my knowledge about the subject. Don't get me wrong, I can answer questions from the textbook (most of the time) but when it comes to doing past papers I sometimes struggle to identify what processes to use especially when it comes to vectors and integration at C4!

Anyway I was wondering if anyone has any advice. I'm sure I'm not the only one who doesn't fully understand the subject, and I can almost guarantee that those who understand now must have started off not knowing at first!