Not sure if I have any tips... But I can assure you that you're not alone. I'm doing my AS exams, and have been feeling exactly the same way. I try and try, but I can never focus... I did well in my GCSEs, but now it's like my brain has stopped working, and can't take anymore. I'm constantly tired, and feel as if I could just sleep forever (however this could be due to late nights, as I'm in boarding and my roommate likes to go to bed late, usually around 12). And then there's the sheer volume of work that I have to get through, and really don't have a clue where to start.
My teachers seem to have faith in me, which is nice to know, they can see something in me that I can't at the moment. In terms of revision, I've been watching YouTube videos and taking notes from them.... That way there's someone talking at me, I'm not completely independent.
I've left it all a bit too late, and there are somethings that I know I won't be able to learn in time, but we have to make the best of it. The worst part for me will be that when my results come out, I'll know that I didn't try my best, and I'll regret it... But at the time/right now, I just can't!
Just remember, whatever happens, it WILL all be okay in the end, even if you delay your plans for next year and retake your A levels. I don't really know what to say, apart from that I really can relate to your situation right now. I hope you manage to to okay, but remember it's not the end of the world if you don't, and nobody is going to think any less of you (and if they do, then perhaps they're not worth knowing) grades don't define you. Good luck xx ☺️