I've recently decided that commitment looks to me like choosing someone, really knowing and wanting them. When I'm in a committed head-space, I don't have ideas of forever after, but I have the sense that I am working together with a loved one towards something quality, something that is worth the effort, and something that I invest in because it's evident to me that it will be valuable as a long term endeavor...
I have had trouble committing to people before because for me, the sense of trust was tied up with it... security, understanding... they all blend together into knowing that your partner is there and really strongly wanting to be there yourself in the same way.
I suppose the crux is intention. I've had lots of cool people in my life that were accidental, experimental, just out of consciousness somehow... but the real relationships, the committed ones, felt more deliberate somehow.