I would agree with Marshallmk1.

I play and sing a lot, it is my favorite thing to do. What I find is that it only works when I have the playing part down cold. If I am struggling with the playing, I can't sing.

I would second his recommendation to try knocking on heaven's door or something similar. I would even recommend you just strum each chord once per measure and get the singing right first, then start adding in more strumming.

Most people get stuck by trying to get too good too early. So please break it down some, log some success, and then improve from there.

Hope that helps.

Jon