My best (or possibly worst) advice is just finish 1 thing, and from there you can't go back to not finishing something. I used to flip projects every other month, now I go years on one thing and don't even want to change.

I say possibly worst advice because there have been times, and I'm currently in such a time, where I'm working on a fundamentally flawed idea but can't give it up in favour of something else. I need to finish it and finish it well, even if it feels undoable.