Personally, this hasn't been too much of a problem for me. As a college student, my projects do extend out longer than I'd like but I am always looking forward to finishing them. However, before map making I used to do a lot of traditional portrait work during which I'd experience this issue.

I think a lot of it can be attributed to burnout or even art block in the sense of finding passion. As someone else mentioned, if I got frustrated with a piece I'd usually try to step away from it for up to a few days before working on it again. Or it might even be scrapped and I'd start over or move to something I'm more excited to work on.

Truth be told, I don't necessarily have the solution to this issue by I do empathize for you. Some things that helped me was only working on things I felt genuine passion for, taking breaks, or trying to find incentive to have a finished piece.

Best of luck friend!