Young developer: How do you keep everything in your head if you can only see about 60-80 lines at a time? I fail to do so and not sure if that’s normal.

I'm tasked with refactoring a giant file of 1000 lines of spaghetti python. I have a good idea of how to clean up redundancy, but keep losing focus because it's so big and I can't see the whole picture in my head to refactor well.

Is this a common problem, if so what are techniques you recommend? If not, what can I do?

Thanks!