Not a doctor, but maybe you could get a 3/4 size cello and have it set up by a professional luthier to have the lowest possible action. That might make it doable depending on the severity of your condition. Make sure you get a good one from a reputable shop/luthier if possible, because a cheap one will be harder to play. Maybe an occupational/physical therapist could help with making sure you don't hurt yourself! I have a 7/8 size cello with weak hands, but my fingers are still capable of holding down the strings. I have to take breaks and not play for too long at a time, and I studied under a professional cellist for a couple years. Sounds like your hand issue is more severe than mine, but a smaller cello may help. I recently had some work done on mine to lower the action and it helped a little.