I understand this isn't exactly what you're asking for as it seems like you burn out on the idea before you even get to the implementation but I've found that for coding projects, keeping my code clean really helps to motivate me to stick with the project.

If I'm building something and the code gets messy I increasingly feel frustrated while working with it which turns me off from working on it - somewhat of a negative feedback loop, so when I start to feel like the code is a hassle to work with, I switch gears from writing features to refactoring the existing code-base.

This gives me a nice change of pace, lets me think a little bit differently than simply "add features", and when the refactoring is done I have a much more thorough understanding of the code I've already written and what needs still needs to be done.

It seems like a pretty simple and obvious suggestion but I can't tell you the number of times I've let a personal project become unmanageable just because I've wanted to keep adding features which at least for me leads to burnout.