Two things.

First, practice the guitar part until you can play it without thinking about it at all, then add the vocals so you can focus on that. It is ALWAYS more important to sing a song well than it is to play it well so your attention really needs to be on the voice.

Second, simplify your guitar part. While you are singing,the listemer's ear and attention are on your singing anyway. You'll be amazed how much you can dumb-down the guitar part before anyone notices what you have done.

Hope that helps.