Yeah, I too, was a singer before I started playing drums at age 10. Learned to sing while playing drums, then took up guitar and bass. Get your part down on bass, so you don't have to think about it, then you can put your brain onto learning the singing while your bass is on auto pilot. Start with songs where notes on both are right on the beat. Lator tackle ones where the singing or bass is syncopated. Hardest one for me was playing the synth bass part on bass to "Very Superstitious" while singing. Even Sting had trouble and had to work at getting his parts right live. Sing every day. Sing while driving. Memorize the lyrics so you don't have to think about them.