I too share your pain. I had a good ol'e cat for almost 19 years....he was my Buddy. He was hefty at about 30 pounds, and became diabetic at 15 years old. I tended to him a lot his last few years with me, giving him two shots a day and monitoring his sugar levels. He was the smartest, most easy going cat ever. He was always happy even when he had troubles getting around. I would carry him places, we look out windows, go out on the deck, help him up stairs and onto our bed. He would sometimes pee without making it to the litter box...and you could tell he felt terrible about being a "bad boy". As time went on I could see that he was beginning to suffer quite a bit even though his mind was sharp as ever. So I made the decision that you had to make. I had to do what's right for him, not for me. As I type this tears are welling up in my eyes.....my heart goes out to you.