This is a very stressful time right now, especially because you are doing your drivers license as well. Nothing is wrong with you.
I did A levels two years back and that feeling that you are never studying correctly or enough is normal. Lack of motivation stems from being so stressed and overwhelmed with all this work. Let me tell you one thing though - DON'T be this stressed over your driver's exam. That should be the least of your concerns when regarding A-levels. I know that you still worry over it - I did as well although I only started my driver's when I was in uni - but you cannot let it get to you. Because look at it this way - if you fail your driving exam you can redo it anytime...if you fail your A levels well that means you need to resit an exam, which makes things confusing when wanting to apply to unis.
Do your best for your driver exam but don't let it get to you - I do think trying out your brother's way is a good idea, get some tips from him. Actually, right before mine a different driving instructor was waiting with me for the examiner and since we had a lot of time left he said we should practice driving before - and from that one drive he taught me how to parallel park much better (I had trouble with that) which helped me do it smoothly before the exam with no trouble.
For bay parking will you need to do it on an actual parking place or within your driving practice area? In my case I only had to show I could do it in the practice area where they had poles set up to help. I would reverse straight while looking back until I saw the first pole from the back passenger seat window where I would start to turn the wheel to move in. When you do this do it slowly to give yourself more time so you park correctly.
F.SA seems to have a lot of useful websites for A level study, I also recommend the Physics and Maths tutor website - past papers are most important. In my experience, making notes did not help as much as going over past papers with the mark scheme.
I didn't sit down and do past papers without looking at the mark schemes, that's time consuming, just look at the question and then answer it according to the mark scheme. It's good that you only have one essay based subject, doing past papers is easier for bio and chem. I also had to try out so many different study methods...and you know what I think rarely anyone ever finds the perfect plan.
Well done for getting all A's on your AS levels! I wish you all the best for A levels and uni...just pull through with A levels, remember only a few months of study, then exams, then summer.