When i started a band i found i had to learn not only to play my parts on guitar but learn backup vocal lyrics,which i found really difficult.My teacher taught me to tap my foot in time to the song,and play along sort of like a metronome,when i found i could do that with ease,she then said now start singing some of the lyrics,not all at first,just a few,and then more as i got used to doing all three-paying,singing and tapping my foot.I found it easier to learn the lyrics first until i knew them off by heart,then i would make sure i knew my guitar parts,and then try and put it together.Even now there are some songs we do that i just can't sing backups to because the parts i play in certain songs are just too busy for me to do both.
Besides I've got 3 other people in my band that can cover most of the vocals and i can concentrate on being the guitar player!!!Hope i've helped in a little way,your other option is just don't sing,it seems to work for Satriani and Vai!!!!!!!!