I can sing and play rhythm guitar
I cannot sing and play lead guitar
I cannot sing and play bass
I played all 3 for about 10 years before trying to sing. I never sang because my choir teacher in 6th grade told me to drop his class and take shop because I was hopeless. So i did
Two very different things to me. When I sing and play rhythm guitar, my strumming is in line with my singing patterns and phrases. I've been told my strumming patterns are unique but fit. When I sing and play bass, my fingers want to pause with my vocal delivery, which is not what a bass line should do, at least not in every case.
Please don't give up. You should explore getting a vocal teacher/coach. I know paying for lessons is a luxury, but if you are serious, you should. Those of us that didn't sure wish we did. My voice is wrecked now, so lessons aren't in my future. Cigarettes and other clandestine things will do a number on your voice.