I had the same problem about 5 years ago. I could fish about 3 to 4 hours, and couldn’t hold a rod after half a day. I couldn’t even pick up a twelve pack without using both hands. I saw an orthopedic guy, and was diagnosed with radial tunnel syndrome. I tried all the stretching and PT to no avail. Over the off season, I had surgery. The surgeon reshaped the nerve and cut the nerve channel a bit. It was the best thing I ever did. I got all the strength back in my arm, and no longer have pain in my hand/fingers. It’s been 5 years now, and all is good. I can fish all day now, and can pick up as much beer as I need, LOL. Best advice is to see a hand/arm specialist.