find out your weaknesses by doing past papers from physicsandmathstutor;
Is it organics? aspirin? physical and buffers? find out what you don't know when doing the papers and going over them with the mark scheme.
Then go over the topics you've struggled the most with according to the papers you've done, you can use YouTube as well if you it's better for you than the book. Allery Chemistry does really good videos for A-Level Chemistry.
Then after going over the topic go do topic papers/questions for those topics - again you can find these questions by topic on physicsandmathstutor.
I joined straight into year 13, I got C/D's in the October Mocks, the a C in the March Mocks, now I feel screwed cus I need an A to get into Uni, but I'm trying and I'm revising. Don't give up, and do your best, worst case scenario you don't get the grade you want but at least you don't beat yourself up for not revising.
Good luck to you and everyone 🙂