Hi Gary,

Singing and playing is rough to get down at first, it took me awhile too. What worked for me was practicing each part separately until I had them down and then trying them together. I think it's a matter of getting your brain to processes two seperate rhythm and melody patterns at the same time, drummers have to do this when they play (without the melody part), playing on rhythm with the right hand, another with the left and another with the feet, etc.
I would try to ease into it by simplifying the guitar part enough so that you can do both at the same time just to get used to it. Maybe strum the first beat of each measure or every other measure as you sing and work your way into it from there.