You don't need to feel guilty. My grandmother, the kindest woman alive, had to make the decision to put down all 3 of her cats. She spent whatever was needed but two of them got feline leukemia and could not be saved and the third peed everywhere and was refusing to eat. He was tested for diabetes just like her other cats - and she kept one cat with diabetes alive for years, until he got feline leukemia. (I'm mentioning all this so you'll know that she was like you).

I am a little angry that people assume when someone puts down a pet, or has to call it quits on taking care of one that they are unfeeling or uncaring. I know that this is not you.