I just about got an A in GCSE chemistry, and my understanding was far from top level. I also took lessons off for no reason in AS level chemistry, which i regret to this day.
In the end i got decent enough grades (A, B and B) in my A2 chemistry exams through hard work and revision materials. If i hadn't cocked up my first year, i could have quite easily ended up with an overall A grade, since first year was so simple.
Point being, with enough work you can gain a proper understanding of any subject. Good luck.