I can't give you advice, but I can relate to your situation and hope my story is supportive. About 20 years ago my cat developed diabetes. I gave him insulin daily for about a year, and he seemed to be doing well. He never complained about the shots and seemed to know they made him feel better. But one day he chose not to eat after I gave him his shot. He developed severe hypoglycemia and had seizures and brain damage. There was nothing I could do but let the vet bring his life to a peaceful end. I still get teary-eyed remembering that day. He was a very loving cat, and I haven't had another pet since, mainly because people I love are allergic to various creatures.
My thoughts are with you.
My thoughts are with you.