I am so very sorry for what you're going through. The same thing happened to me with my sweet Bullwinkle-duck a couple years ago. It was heart-breaking. I took him to a regular vet a few times, and even to an avian vet who took xrays, did bloodwork, etc...no cause found for his issues. Gave him all kinds of medicines...dewormers, steroids, antibiotics...nothing helped. I even built him a rolling walker (similar to what babies use) which he hated. I struggled with the same question - is he alive for his sake or mine - because he was still very lively and had a great appetite. I finally got my answer when he could no longer swim - he would just roll over and nearly drown. I mean, a duck that can't fly or walk, okay, but he can't swim? What's the point in being a duck if you can't swim?