Unfortunately, not doing past papers will catch you out. This time last year, I did two solid days of revision in half term (like, ten hours a day) for Biology and got a D in my mock retake. I know a kid who did loads of maths revision, but ended up with 7/72 marks in his mock.
Don't give up, there's still plenty of time to turn it all around. Start doing past papers, and you'll start to get higher grades.