If I'm working on a project I don't have external motivation for, like a personal project, etc. I'll make sure I stop working only when I reach parts that are fun or at least clear. Next time I sit down, I want to either look forward to jumping back in, or not have to sit down and deal with a super weird edge case that had me hating life earlier.

This also applies to lots of things, you can quit, just do it after you made progress, regardless if it's shipping v0.1, running a marathon, etc. Don't quit when you're sad, get to a spot when you're happy then quit.