the left hand is the weak part of the position...the right hand is getting exercise...Perhaps you are pressing TOO hard because the felt is worn and requires extra pressure. Depending on the autoharp, it can be possible to change chord bar arrangement (21 bar is easy,,,recutting them on a 15 bar is harder)...after 30 years, I 'can' still cramp my left hand & wrist with excessive playing.

Do consider playing it on a table or your lap, although Brian Bowres style is harder that way...but if you can find a Kilby Snow record, you will see what CAN be done!!!!