I wonder if a lot of your pain is not a repetitive strain injury-caused by having your hand/fingers in the same position for long periods. I had a similar problem years ago caused from holding a putty knife/sanding block/paintbrush on a house project. The hand specialist put me on anti-imflam's with the result being worse than the pain [GI] My chiropractor [who I saw regularly anyway] said let me try something, and over several visits helped a lot!

Stretching your fingers, resting the hand, not "popping" the joints will help too. I mention this possibility because part of the problem could be non-joint...

But I also agree with Genie- glucousamine/chondroitin/msg is good; lots of water to keep joints hydrated is good; stretching/working the fingers in non-guitar motions to keep 'em limber; you might also read articles on the 'net- there are doctors/therapists who are "up" on the connections between playing an instrument and arthritis pain.
The subject has come up several time on the autoharp list...

'Hope you get better soon!