I could never get a doctor to give a precise answer to that, just, "some kind of repetitive strain injury." It seems to have had psychological components to it also: I was so worried about it that I created additional tension there and never let it relax, and I was always super stiff/tense while using the computer. So whatever new ergonomics or exercises I tried, I continued clenching/worrying whenever using a computer. And then I could see that the tension was making things worse so I'd get worried and more tense because of that which created a bad feedback effect.

Getting better was partly just learning to not freak out about it—the situation was manageable and I wasn't going to have to give up my favorite activity that I'd spent so much of my life training for etc.

More exercise, better ergonomics, and taking breaks also helped. I've been practicing meditation for a couple years now too, and it's finally been getting to the point where I can use what I've learned to see and modify the bad mental patterns I established during 10 years of unpleasant computer use (including getting out of the feedback effect mentioned above).