KEEP NOTES. Write everything down. Write down what you were thinking about, how you felt about decisions, things you tried for bug fixes. you never know when this stuff will be useful to you again.

I keep really basic markdown notes in a git repo, roughly one file per day with an ISO date name, but some things I give a separate file name (still with the iso date, but with some descriptive words) for things that might be good to re-access or are part of a series of similar notes (like a TIL file or a recurring meeting with the same person).