Maybe you need to examine what your true goal is. Why do you want a side project? Are you doing it because you feel like you are supposed to have side projects, or because you actually want to accomplish a goal? If it's the former, you could try to align the side project with another goal of yours - say learning some new language or framework.
If you are just trying to build a portfolio you can come up with a (small) "SMART" goal, and commit to completing it. After you reach the milestone you can make a conscious decision whether to continue or not - but either way you have _something_ completed.
> Maybe you need to examine what your true goal is.
I think that's absolutely the most important factor here. People don't finish things for a variety of reasons.
Perfectionism is a big one. It's tied to a fear of failure that manifests as losing interest. If the root cause is perfectionist tendencies, one thing that can help is to collaborate with others. Just doing something for someone, helping them out in some way will often de-fang the perfectionism and self-criticism and drive you towards something tangible.
It could also be boredom. If you're doing X at work day-after-day, maybe it's not a great idea to also do X as a side-project?
I personally am allergic to SMART criteria-- too much association with corporate performance evaluation fuckery, it kills motivation and creativity for lots of people.
If you are just trying to build a portfolio you can come up with a (small) "SMART" goal, and commit to completing it. After you reach the milestone you can make a conscious decision whether to continue or not - but either way you have _something_ completed.
https://en.wikipedia.org/wiki/SMART_criteria