borjacobs,
I've been in a few bands in my time and I know what you mean about people not able to make the same level of commitment. This is something you probably have to establish before joining or starting a band. Not only do you need to know what everyone wants from being in the band but also what the musical direction will be. Might as well get that figured out from the beginning rather than joining or starting a band that is destined to break up eventually due to musical differences.
As for contributing to songs but not starting and finishing songs, that pretty much describes me too but I've found plenty to keep be busy by collaborating on songs started by other people. If you were to search all of the music I've posted here, I'm never the songwriter, but a contributor to other's songs. I feel better able to do that, than to write my own songs. That may change some day, but for now, I'm doing what works best for me so perhaps, like me, you need to partner with a songwriter, but that's no reason to stop trying to write your own songs.
Good luck.