Thanks, good luck to you on finding a completely pain-free solution!
This is the late 90s, so neurologists and other doctors weren't very useful at all. One even suggested surgery to correct my problem which got me really scared I might be out of the profession and wondering what I could do.
What I found out was that there's a nerve that runs all the way down your arm into your hand/wrist, and when I twisted my right hand outward to use the mouse, it was pinching this nerve constantly. But instead of having pain in my wrist or forearms, it was causing pain in my neck and shoulders. This is a well-known phenomenon called "referred pain" where the body might not have the proper pain receptors to show pain in one area, so it sends the pain signals elsewhere.
I started to wonder why I could write notes with a pen for hours and hours throughout college and never had any issues, but a couple of years out of college, I had such debilitating pain that I almost had to quit. So I bought a mouse that had the form factor of a pen and tried to recreate my experience writing by moving my keyboard away and using the pen mouse the way I would when I write with a pen. It was a very primitive mouse with a roller ball the size of a marble at the tip, but it completely worked. After the pen mouse died in 9 months, I tried to recreate my wrist angle in a similar way as the pen mouse with a normal mouse and it worked as well. As I said, I've adopted this convention and people comment on how weirdly I hold the mouse, but I've been pain free ever since.