I have a friend who has no fine motor control of her right hand because of a stroke she had in college. Her left hand suffers greatly from overuse syndrome. However, she loves music and is very strong willed. She now plays both piano and organ with her left hand only. She uses one finger to pick out the melody and the other fingers to play chords. She sounds very good and I'm very proud of her.

I also concur with those who recommend the lap dulcimer, although it requires some use of both hands. You can put it down on the table, use a noter, and strum. It has a lovely sound which may well appeal to you as a string player.