Hey there! I sat my OCR A-Level chemistry papers last year so i've been there and trust me when i say i feel you.
Have you tried youtubers? Do you learn from videos? I remember watching 'Allery Chemistry' on YT a lot nearer the time because he explained the concepts well and went through some exams questions. I think he covers all exam boards separately too. Also, regarding markschemes, make little flashcards on anki with bullet points from the markscheme and memorise those answers. I used Physics and Maths Tutor for practice questions i think btw.
You say you struggle with time? The solution for that is completing your papers and practice questions under timed conditions. It's all about exam technique. Know when to skip and come back to questions later. Get those easy marks first - that'll give you a confidence boost and then while you get warmed up on those, you might think of something to add to an answer for a harder question you skipped earlier for example. I'd say slow down too. Take a deep breath, oxidise that adrenaline if you're panicking. Read the questions properly. Don't rush because you're prone to error then.
Hope this helped at least a little bit. I believe in you. You'll get that A, just keep at it!
Best wishes!