I can't sing and play guitar at the same time?

Hello there. I've been playing guitar for about 5 years, but I struggle to sing and play at the same time. I'll be playing, and I'll try to start singing, and my timing on my guitar playing goes out of control and it sucks. It would really make me happy if I could sing and play at the same time. I don't even sing great, but it still saddens me when I can't keep timing when singing and playing. Can you help me in any way? Like, an excercise that I can practise when I play? Any help will be greatly appreciated.