Singing (the way out)
As explored in an earlier post (see Chopsless), when writing/producing music I often end up with an instrument in hand that is not one that I feel particularly adept on. Guitar is one example. I've spent time with guitar over the years but as Leonard Cohen said, "All guitar players have chops. Especially professional ones. But I have only one chop." Marginally convinced I have even "one chop," I want to share one solution that might counteract the frustration brought on by a lack of instrumental fluency when composing music. Sing.
It does not matter if you are a singer although this approach is perhaps most relevant with composition that happens through some type of recording set-up. Sing along with the track. If there's a beat, a bass line, a melody, or chord changes, record some sketch of those and put that recording on repeat. Improvise vocally until you find the part you want to hear, and then translate that part to the instrument on hand. You do not need perfect intonation as a singer; you do need to be able to find the notes you are singing on the instrument at hand.
Singing bypasses some of the "I can't play it" chatter and cuts directly to a musical statement. I find it also makes for interesting musical ideas as the ideas are not based on muscle memory, however limited. This approach is more challenging for the first instrument in a composition (i.e. no other parts to sing along with), but it can still work. Alternatively, you could lead the composition with an instrument you feel more confident with and then circle back to one that is less comfortable.