I've never really been able to sing while playing guitar, is there any way to practice this?
Some songs are just easier to do than others. The toughest song I ever had to play bass on and sing was Sweet Emotion by Aerosmith. The vocal line and bass line are doing two completely different rhythms.
I had to learn each part individually to the point I could do them on autopilot. I finally got the bass part to the point that I didn't have to think about it, and then I could just worry about getting the words right.
As the other posters have said….practice. I would just say you might want to practice the playing and vocal separate for a while, and then put them together.
Good luck.
Greetings from Austin, Tx