I don't wanna brag but I personally know a few people who started revising even later than this and have achieved 95 UMS average in all of their AS's. They did however learn that they won't do the same mistake at A2. I'm sure everyone underestimates AS, me being guilty of that. You have enough time, even more than enough. If you fret and worry then you wont get any work done. Think of it like this, if you think you are not going to get high grades then you arent, but if you think positive, chances are you'll put in the effort to get good grades.
Stress is natural at this point, you don't know how scared I am for these exams too but just calm down.
I don't do any similar subjects but I have friends that do English Lit and History and they seem to do past papers and read alot...