I suffer from anxiety too and you sound exactly how I did before going into Year 13. Honestly, it sounds like you're doing a lot already but I know it never feels like enough! My advice for you would be to do as many past papers as you can and anything that you struggle with on those, THAT is what you need to use cue cards/mindmaps etc. for.
Doing past papers helps you with recall, essay structure and general practice which is super important. It's so important to understand where the gaps in your knowledge are so don't be afraid of answering past papers without a full working knowledge of every aspect. I also found it really useful to teach others about topics, as this solidifies the information in your brain.
With cue cards, these are mostly useful for figures and dates, but don't rely on them too much!
My timing was awful too, History was a huge struggle. The best thing to do is to look at all your past answers and highlight anything that you think was unnecessary and that doesn't answer the question - you may begin to see a pattern and then work to avoid this and reduce the time it takes to answer. For me, my History exam answers usually involved a lot of me showing off the knowledge I had instead of answering the question. A good answer doesn't need to be long, just directly linked to the question.
Make sure to give yourself a break too! I used to have a rule that I would never study past 9pm so that I could get enough sleep and could do something enjoyable for the rest of the night. I know that the coronavirus situation is making everything much more stressful for your year but when applying for a Masters in 2023/4 they will not be looking much at your A Level grade but your Uni grade.