Hi excelsiorxlcr,
It sounds like you are putting a lot of effort into your studies, so I think it's good that you are being mindful of getting burnt out.
Losing concentration during exams is very normal, so try not stress too much about that (easier said than done, I know). There are, however, things you can practise to try and minimise loss of concentration. One thing I recommend is when doing practise questions, giving yourself 10 minutes to dedicate completely to the question, and once that 10 minutes is up, you can stop focusing so hard, and go and do something else. If you practise this a bit, try and each time concentrate 5 minutes longer than you did the last time, and this can help build your stamina up.
Another thing I recommend which can be extremely helpful, is giving yourself time each day to exercise. Getting your heart rate up and thinking about something else for 30mins to an hour can really help hone in your focus when you go back to studying. Even now at uni, if I haven't exercised for a couple of days I really struggle to focus, so that's when I know to give myself a break.
You're also doing the right thing by meditating which can also be helpful. I think the key is to go easy on yourself, and don't get frustrated if you are struggling to concentrate, it's very natural and it happens to everyone. I also think you'll find that when you are in the real exam, your adrenaline will be pumping the entire time, making it very hard to lose focus.
I know these tips sound basic, but it's really the basics which I find helpful when I'm getting stressed/burnt out.