Here is some things to keep in mind...
...The first thing to do, whether you are playing your own song or someone else's is to adapt the guitar track to the voice (a capo if needed). Meaning, no solos while you're singing. Also watch out for picking. You may not, in certain instances, be able to pick and sing simultaneously. You can't play as much as would like, so the other musicians in the band should cover your guitar parts as much as possible so that the song doesn't sound too empty.
Of course, and I know you knpw this, but it's a simple fact that the more you do it, the better you'll get at it.
Learn to play the guitar part so well that you can do it in your sleep.
Then learn to sing the song so you don't have to think about it, without the guitar.
I would suggest merely humming the melody or "scatting" (da da dum ti dum)or whatever gibberish comes out of your mouth at the time to the melody while playing through the chosen chord progressions on guitar. Otherwise you're having to concentrate on the words as well as your playing simultaneously which is where lots of people who have only played as an accompaniment for others without singing along themselves get frustrated.
Then after you've mastered both versions, then combine the two and it should go pretty smooth.
That's always worked for me.
Start simple. Better to be simple and right than complicated and not right. Don't ask your fingers (or your voice) to do more than they're capable of--and they will become capable of more as time goes on. Have fun. It'll be fine.
Also, recording yourself so you can really hear what you sound like can be terrifically (or terribly) enlightening.
Add some point, begin to practice to a metronome or other steady beat. (It may seem hard at first, but will be good for you in the long run...)
The key is getting really comfortable with the guitar. When you are beginning to play the guitar, it takes great concentration and a fair amount of physical discomfort. That all goes away in time. Once the playing becomes second-nature, you will be able to sing along. Trust me.
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