I don't like to sing, but with some effort I can manage it while just strumming chords. When I try to do anything complex, the lyrics just go out the window.

As with anything else, the bottom line is that you just have to practice.

Whenever I've tried to learn a song with the lyrics I've found it helpful to play and sing along with the CD or tape. This way you don't have to think so much about what the lyrics are and you can concentrate on your guitar part more while singing.

The other option (this is the option I would recommend) is to just remain a guitar player and find and team up with a lead singer.