My musical regret is lack of discipline.
I learned how to sight read music when I began playing piano at age 4. When I was 8, I switched to an accordion with 60 bass buttons and a keyboard for the right hand, and gradually worked my way up to a 120 button beast. At 12 or so, I switched back to piano. But at around ten, I began to lose interest in reading and interpreting music, and started coming up with my own ideas. This may have been due to a combination of interest in exploring my own musical ideas, and sheer laziness. I didn't like practicing all that much as a kid. I did it; at some point I discovered that I could learn a piece by ear after hearing the teacher demonstrate it, and could fake my way through a lesson. What I liked doing was playing music I dug. I was on the piano or organ (we had a Hammond) constantly, but not "practicing" in the true sense of the word.
It was more like semi-educated putzing around.
My sight reading skills certainly declined as a result. And I could only take complicated pieces so far before my laziness was discovered by my teacher!
By the time I was in high school, well, there were girls to flirt with, and buddies to hang with, and books to study for school, and did I mention girls? And bands? Rock music required about 10% of the skills I had developed by 6 back then.
So on piano, I'm certainly fast, even highly skilled, but not a virtuoso. Virtuosity requires dedication and the ability to rise to the challenge of reading difficult music. Thus, I only got to middlin' level on Bach or Chopin stuff, nowhere near concert level material. On guitar, an instrument I didn't pick up until I was into my teens, I'm largely self-taught. Though I'm what "civilians" would consider a very good player, any guitar player listening would realize that I'm not virtuosic.
I guess you could say that I developed a "composer's" piano and guitar skills. I suppose that's fine, as I'm a composer by trade.
But I wish I'd had the patience to develop my chops earlier in life; there's been some research to show that unless one develops very substantial skills on an instrument by the time one is entering puberty, virtuoso skills are rarely attainable. You can get very, very good, but not truly great. This evidently has something to do with how the brain develops neural pathways.
So my regret is that I lacked the discipline to reach my potential (not saying I'd be great, but I'm pretty good with half-trying, so there was room for improvement) on either instrument. Have you got a regret or two about your own music?