I like the idea of having an “end of life wrap-up" for half-finished side projects. Rather than just stopping and leaving them abandoned, you make something like a report on what you learned, what you built, and why you're stopping. Then it feels less like you've abandoned something outright.
I do kind of the opposite, every week every project needs to justify why I should keep doing it and what I learned recently, and if I can't come up with any good reasons or good learnings, I abandon it.
That's a good idea too, but I think the wrap-up postmortem helps me clear my mind a bit. Personally I feel like having a formal declaration of "it's finished, for now at least" takes a weight off my mind.
Also it can give a feeling that it was not a waste of time - lessons learned, what you would do next time on other projects, other avenues to look into.
For years I wrote a technical blog intended just for my own reference, as the small effort required to write it up, create images and so on felt good. It was also a good point to think about what I had _actually_ done - sometimes this made me realise small mistakes or missing details.
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