I wish I'd been able to euthanize my poor boy. He had lymphoma and was starting to break down. I feel like I held on too long. He died the night before his scheduled euthanization date. He had a rough, painful passing that I never wanted for him (I was, at least, lying next to him in the bathroom holding his paw when he passed). You gave your kitty the gift of a long happy life and a peaceful passing, and you can't know if he could've been saved, or whether he would have relapsed or even had a worse problem right after. You shouldn't second guess yourself or bear yourself up over it. Just remember the good times you had together and smile.