Yeah, a discipline I wish I have. Still, I'm getting better slowly.
You might find holding yourself more accountable for finishing them to be a motivator. If say for instance you were going to blog about your progress daily regardless if you finished or not.
Anytime I think about it I think of all the endless stuff I need to work on my main project.
That's actually a good thing! Jams are better suited for those who don't have a main project to worry about or got a burned out of it and need a kind of break that doesn't rust their gamedev skill.