How do you guys prevent burnout when working on a side-project? I'm probably not the only one here, with multiple unfinished projects.

Not every project needs to be finished. However when I do want or need to finish something I remind myself of why I wanted to undertake this and what I will gain from it.

It is all project Management. understanding that things take time, prioritization, etc.

I agree with @gitblame, not all projects have to be finished.

For any side project, if you can I would say start with very minimal requirements and make a simple MVP so you can complete the first part of it and at least feel accomplished in that way. And then you can just iterate and add new stuff after that.