First of all, don't get stressed about it, because as you said, it is very hard and it is something that everyone has trouble with at a point. Basically, you have to learn one part, either the singing part of the playing part, inside and out so that you could do it in your sleep. Then once you get to the point where it comes naturally, try adding the other factor to the equation. I recommend starting with a simple song, like blowin' in the wind (chords: verse- G C D G chorus - C D G C) or time of your life (greenday)- G C D .