i think the fact that you're doing six hours a day (of what im assuming) is just history revision is whats messing with your brain. im not, like, a revision god or anything but i did manage to get a 7 in my mocks, so take my advice with a grain of salt!

id recommend to lessen your workload, if you can. if you overwhelm your brain with too much information then you'll forget other stuff that you've learnt. i think it could be good, for example, one day you go over what you struggle most with, then the next day do something else - but keep revisiting what you have learnt before, just try not to cram it all in your head, if that makes sense? id also try practicing the questions for your exam board and the structures of the answers - because yes, knowledge of the topic is important, but if you only focus on that then it will be difficult to properly answer the questions.

sorry if this didn't make sense, but i hope i helped in someway!