A-level is like the stepping stone to university. It's optional and you only do subjects you like. Suddenly you had to actually understand the stuff. It wasn't just a memory test. The first exams I got B all around. I wasn't happy with this so I started doing homework etc and actually understanding things.
Coincidentally, my breakthrough also came with Newtownian mechanics. I was quite good at integration, but I didn't understand why I was doing it. Suddenly the whole thing started to make sense when I realised it was about rates of change etc.