Though probably not at all what you're expecting... The TLDR version is old habits die hard & I've been fighting an ongoing struggle to overcome them.
So, I'm a klutz & always have been(though that's not directly what this is about), I can't dance _at_all_(which may surprise those of you who know I'm a musician, but again isn't directly the problem). I'm also adhd, which may be partially related to the problem.
The problem its self is directly related to playing music, but I think it's a combination of neuromuscular & psychological issues. To make a mechanical comparison, I feel like my muscles function more like solenoids than linear actuators. Does that make sense?
The concept of "smooth" has never been anything that crossed my mind at all, with 2 exceptions: autocross & billiards - and for some reason, once someone explained the concept of smoothness to me in each situation, it just kinda stuck(unfortunately I get bored with billiards after a couple games).
What I've realized over the last couple years is that lack of smoothness has caused me to go through repeated cycles of practicing music a lot/getting frustrated about not being able to overcome my problems/quitting playing for months(or years)/rinse & repeat.
I do think part of the problem is related to my adhd, because in band/ensemble situations when I'm diverting some attention to everyone else, my playing is less inconsistent. Although I'm often playing less complex/challenging parts then too. The net result though is it's always a struggle to play through any piece of music correctly & consistently.
I've dabbled with yoga & meditation over the years, but found yoga to be a combination of boring & painful, and it's truly a painful struggle to shut off my brain for even 5-seconds of meditation.
I'm basically always talking to myself, and I've realized that's often to my own detriment in autox. Though rarely when I've had "silent" runs, they've often been my best ones of the day.
I am on adhd meds, though I only take them on workdays. It helps me focus & be more productive at work, but I've tried taking it on weekends before gigs & it had no impact at all. Fwiw I'm not a smoker(of any kind) and not against it - as a musician I've certainly spent plenty of time in a cloudy haze of cigarette and/or pot smoke, but even then it never changed my playing noticeably.
So ultimately, I feel in order to move past this barrier I think I need to completely reprogram my body mechanics. I've tried tearing down my playing & starting from scratch with the basic rudiments, but my(admittedly somewhat odd & unique) playing style just returns on its own because it's an internal part of myself. Unfortunately I've not figured out a way to reprogram mysel successfully yet.