I got a U in History on one of my essays in December during my AS year. In August I got an A. Anything is possible
Got U in both my economics mocks twice (we did the mocks for each unit twice hence four papers), did the real exams and got nearly full ums, I have to say though that I just didn't take economics seriously enough until easter where that was the only thing I was studying.
A few years ago when I did GCSES. My mock English literature I got a U. I didn't bother revising for the real ones as I was bloody lazy lol and came out with A* so it is possible. All the way through year 10 I was getting U in biology, chemistry and physics and I got 1 C and 2 Bs good luck!
I got an E in my Chemistry mock and I need AAA to get into Uni...
Chill...a chapter a week then loads of questions at the weekend! Repeat until you have covered the content.. then bomb out the past papers a month/ 2months before the exam.
I got Us in various Biology mocks and got a B overall (a mark of an A annoyingly) I am sure you can do it as long as you put in the work ofcourse.