So sorry to hear of this problem. It certainly reminds me of what one of the banjo greats, Tom Adams, contracted a few years ago, called focal dystonia. His index finger became erratic, to the point that he has chosen to play only two-finger style in public. This was a serious loss for the banjo world (Tom quit his illustrious lifetime career as a full time player, and got a "straight job" for the first time in his life), and of course a cruel blow to Tom himself, though he does do some public playing still.
I can't tell you details on this, but I have a friend who got focal dystonia and unlike Tom, managed to beat it with a sustained effort over a long period, and maintain his successful career performing mostly on fingerpicked guitar. If you've not heard of focal dystonia, I guess the thing to do would be to "google" it and check it out as thoroughly as you can, to see if that may be the problem. If you have already done that and feel stymied, I would be willing to put you in touch with Joe, who is a nice guy and would try to be helpful. But I'd say first thing would be to explore the available literature to see if f.d. matches your symptoms. The condition is apparently pretty obscure, so it wouldn't shock me if three doctors hadn't heard of it. Also, the problem for both Tom and Joe was I think limited to the index finger, so maybe that means it's something else. Just guessing here, obviously.
I'm sorry I can't be of more help. I wish you the very best with this problem. You know, we all lose our abilities at some point or another, especially thanks to just aging. Saying goodbye to abilities (and people) you've been attached to for a long time seems to be one of the hardest parts of life. But we all have to do it; it's inevitable -- some sooner than others. I humbly offer you the suggestion that you might find solace in all the time you were able to play in a much more satisfactory way, if in fact you no longer can do that. Undeniably, there is still a lot you *can* do, and I hope that if nothing can be done about your condition, there is a great future for you doing something from that set of things that you still can do well and with pleasure.
I wish I could offer you more than these words, but that's my best for now.