Somehow I missed this. Apologies!
To be honest, falling behind is natural. It's usually really hard being on top of everything for the entire year. When I did maths in high school, usually there were bits and pieces that never sank until later on, or that I had forgotten until I started studying it again. It wasn't always because of out-of-control factors like fast pace, but they definitely do impact.
How you combat this is really up to you, but these were some strategies I employed:
- (If I felt lazy:) Literally just say "screw it I'll come back to you later". This strategy works sometimes, and fails at other times. It doesn't work when the stuff at that point in time was necessary for something that was about to come up, say, next week or something. But if I knew I wouldn't have to use that concept in the near future, I would just wait it out a little. (Of course, right before an exam though, I'd make sure I studied heaps out of it.)
- For now, gloss over. This is sort of like the above option, but with more work put into it. At the very least, I'd try to skim down the important key concepts, and focus on understanding those instead. That may or may not mean understanding worked examples - I'd squeeze in examples if I felt my brain actually could handle it. The advantage of this is that if I require said concept that I don't know well yet, at least I know barely enough to not fall behind in class.
- Lag behind at school. Sometimes, this requires that your teacher isn't overly-strict and is willing to let you practice maths at your own pace in class. But it really just says that you absolutely need to focus on what's currently the more pressing issue. Although you may sit down in class and copy down what your teacher writes on the board, you choose to use any remaining class time on doing things at your own pace instead.
- Devote more time to catching up during weeks when you're
not swamped. Basically, catching up is important, don't burn yourself out!
- Jump some questions. I generally advise against this (at least, in high school), but sometimes a bit of less practice in the
short run can help you catch up on concepts more quickly. (Please don't do this in the long run!)
A mix of the above is also healthy.
These are just loose strategies I have off the top of my head. If there's anything else you feel is more specific to you, I can try to expand on that. Otherwise hopefully this serves as a start.