you are talking about 2 different parts of the brain and its a matter of training...not an easy task at all.
For me I could hold a great rhythem and god for bid I opened my mouth to say ANYTHING let alone singing....I lost my rhythem...evem if it was only a 3 chord pattern....you just have to start small
Try doing something like two chord changes....or heck even one chord if you need....and start by trying to hum or just talk....maybe sing the alphabet. Right now it doesnt have to be the best thing in the world...just let your brain know that it needs to get the two areas working together...youll get there.
like with all of it....practice and time
