As I'm studying Greek these days, I'm reminded of the importance of having a goal or deadline.  I went to Greece completely by myself at the beginning of July, and it was a real motivator to know that soon I'd be thrown in the deep end where I'd be surrounded by actual Greek speakers chattering away at the speed of life!  If there weren't a date circled in red on the calendar, I might have been tempted to noodle along without particular direction instead of doing the hard work it takes to develop a new skill.
 
So if you want to play guitar more when co-writing, maybe it's time to make a date with a lyrics-only co-writer to push you out of your comfort zone. Or book a writer's night without bringing a guitar-player to light a fire under your butt to practice and prepare to accompany yourself.