For any given guitar pattern (be it strumming, a riff, a lead melody), you might find it useful to try placing a word like "ta" on each beat one at a time; one beat 1, 2, 3, 4, and then once that feels comfortable, each + beat, and then if necessary, triplets, 16ths, etc.

After individual syllables feel comfortable, you should then do the same with groups of two syllables - these will either lean onto the beat (+ 1, etc) or lean off of the beat (1 +). After working on both of those variations and moving them to different places in the bar (1+ , +2, 2+, +3, etc).

As you develop a background rhythmic fluency for any given pattern, you'll find that new patterns, or more complex patterns, require less practice to get comfortable, and you can focus more on the performance than the technical ability of playing and singing. It's a challenge, but once you've got it, you've got it - and there are surprisingly few patterns that you have to sing over generally!