I manage to keep my main side project (~20,000 lines of code) in my head despite not meeting conditions #1 (I'm always distracted), #2 (I work in my free time a

I manage to keep my main side project (~20,000 lines of code) in my head despite not meeting conditions #1 (I'm always distracted), #2 (I work in my free time at home), and #3 (C++, woo) because I have extensive documentation about the why of code, and this helps me put myself in the same state of mind I was in when I wrote it. It does help that all the others apply to me, though. I've rewritten it kinda twice, I already mentioned my comments, I work basically alone on it, so nobody else edits anything, and it started out very small.