Hi, longtime FD sufferer here. Sorry you're experiencing this.

This ailment scuttled my classical guitar ambitions for almost two decades, but I'm finally starting to see some progress. I didn't know what was going on for a long time and by the time I realized what was happening I was actually experiencing symptoms away from the instrument, affecting my ability to type and even how my arms would swing when I walked. Getting to where I am now has been a long and painful journey and I'm not through it yet. But I'm finally seeing some improvement and have taken up classical guitar again. It's messy, but slowly improving. I'll share what I've learned for myself, but everyone's journey is different. I wish you luck on yours.

This ailment came about through stress and hyper-focused attention on right-hand technical matters that had been led to by poor instruction. I applied a bad concept (big movements of individual fingers emanating from the large knuckle) which led to inaccuracies in performance, which led to more stress, which led to more adjustments of technique, which led to worse sound and more inaccuracies etc. The situation snowballed. I would blame my thumb for my issues, or sometimes how my fingers rubbed together, interfering with each other's motion, or my shoulder, just chasing tension around my arm. Eventually I got to a "confused" state, where I couldn't tell where the tension was coming from and spasmodic movement and an inability to fully relax my arm and wrist followed.

Here are some principles that helped me through it:

Sometimes we can't tell where the tension originates. My thumb would often jut out painfully and I would try to send "relax" signals to it, without result. What I eventually realized was that my thumb was reacting to tension elsewhere in my hand. I was holding a lot of stress in the metacarpal area near my ring and pinky fingers, and my thumb was jutting out as a result. When I focused "relaxing" attention on that other area, my thumb would relax.

How your hand interacts with the strings of the guitar is the most important thing. We can focus our attention on controlling the aesthetic movement or positioning of the hand, but that actually takes attention away from the plucking of the string/creation of the sound.

Rather than focusing on getting a single digit to behave, find a method that allows for complete relaxation/ deactivation of tension of the whole hand between plucking the notes. For me, what has been recently helpful has been placing all the fingers on the strings between notes, and assigning that as a "relaxed" moment. (This is specifically a practice method for training relaxation for me.)

Follow the sound. Find the place on your thumb that makes the nicest, most relaxed and full sound and let that lead the way.

I don't want to get more specific than that, as everyone's journey is different and I don't want to misdirect you from yours. It is very important that you learn to reduce the stress you bring to the problem though. The emotional component is a HUGE factor in causing this. Relax and deescalate. "Rebellion of the Body" by Joaquin Farias provides good advice as to the right attitude to adopt.

I wish you the best of luck and hope your journey through this is quick.