Past papers.
The first time round, I got a D. I retook the second year and 1 of the AS exams and got an A. Both times I made neat copies of my notes and used flash cards. But i couldnt answer the questions.
I printed every exam paper from my exam board, as soon as I'd finished a topic, I answered all of the relevant questions and marked them. Once we'd completed the whole unit, I did papers again. I kept my books open, but wrote in different colours. If I answered without looking at any info, i gave myself bonus points and set a target. If i reached it I rewarded myself, with a movie or id meet my friend for coffee, then the next time I had a higher target.
I think it helped. By the time I got to the exam I was bored with answering questions. The examiners might change the style of the question but they're pretty much after the same information each time. It felt like a routine.
The othet thing i did was if i didnt understand a question after reading it 3 times, i marked it with a little star and moved on. More than a few times I'd be thinking about another question and id remember what to put for the one I couldn't do.
Chemistry is one of the hardest A-levels. But you can do it.
Good luck!