A theory -- It's possible that your original idea is pretty seductive on
the surface, and then you dig deeper and realize it's not that compelling.
Something else comes up that seems more attractive, and you jump on it.
In the meantime, you forget why the first idea wasn't so brilliant, and go back to it once the challenges of the second idea become apparent.
So, next time you leave your first idea, write down why. Then tape that note up on the wall so that next time you're tempted to go back to it, you won't.
I hope you don't have the same problem with bad boyfriends...
Also, building on another answer is a "commitment mechanism". I've told everybody I meet that I'm doing "X", so that now if I change my mind, I'm going to have to explain why I gave up. I've put more pressure on myself to stick with it unless and until I'm forced to stop.