I used to (and sometimes still do honestly) suffer from this. What I've found helped me actually get finished projects out the door was participating in game jams. Really short ones, mind you. No more than 3-4 days.

It not only forces you to stick with one idea, since you've no time to switch to a different one, but the timescale is short enough where you can see the game come together relatively quickly.

The due date, I think, is what helped me the most. Having a date and time when the game was due, almost like some kind of homework assignment, really helped push me to the finish line.