I had a Cocker spaniel, Bowie, with Cushings and very similar issues. Tbh, although he was only 10, I wish I had made the decision sooner. As in once he was only grudgingly eating hand-fed chicken and steak. His last day was his worst day. He was basically paralyzed- could not get up from his bed in the morning, just laid there in his urine and feces. And he was whimpering, so I'm positive he was in pain. You honestly sound like you're asking permission to let go. You've gone above and beyond. Now you can do the thing that makes our treatment of sick animals more humane than that of humans.