A levels is just as much about learning material as learning how you learn. I know in hindsight the best way to approach material, and you could learn a heckuva lot of it, but still nowhere near enough to be exam-ready.
When you go to A-levels, it's like your mind develops to up its game, but that's not instantaneous. Like I remember so many people finding the basic n = m/M equation very hard at GCSE, but that's child's play at A-level, becaus eit forces you to think more clearly.