I say, force yourself to finish songs, no matter how much you think they suck.
I go through stages with each song:
1. Jot down some ideas and think I'm off to an awesome start
2. Start fleshing out the song, but start to think, "WTF, this doesn't sound good at all"
3. Stick with it, despite my misgivings, and get to the point where I start to like it again and enjoy working on it.
*shrug* So yeah, just force yourself to turn your ideas into full songs.