I suffer from this. I even get as far as nearly finishing a project, and then just decide to abandon it as the last 20% is just mentally exhausting, and I should have written the whole thing in language X using framework Y…

Haha that's the legendary 80/20 curve of a sidegig ;) As i grew older i just do the work when i feel it. When i don't feel it after a few days i stop working on it. The feeling may come back but i won't force it like my younger self would do.