I think you need to work on forgiving yourself. You did the best you could at the time. The last cat I had was sick and lathagic, too, but I didn't notice in time. By the time we got him to the vet his kidney's were failing and we had to have him put down. I blamed myself. All the if only's couldnt bring him back. I still have pangs of guilt when I think about him. If only I had noticed sooner that he wasn't feeling well. But I didn't. I still cry when I think about it, but it has gotten easier with time. You have my sympathy. And it will take time for you to go through the grieving phases but you are not a bad person.