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.
Another way to avoid that feeling is changing your mindset around what's "abandoned" vs "completed". "Completed" doesn't have to mean "published project and made it FOSS" or whatever, it could literally be "Scratched an itch to play around with library X's APIs" or something, or just "Wanted to see if it was possible".
Nowdays I "complete" every single of my side-projects, some of them in some hours, because "completed" no longer has to mean "it's public and people can use it", mentally this feels a lot better :)
I think, we're in agreement : it's your project, so you get to say what 'completed' means, but my criteria is usually writing some small amount of text about it, even if that text is "this didn't work, ho-hum".