I fully understand your pain as I have gone thru similar things. I had a dog, whom I looked upon as my younger brother. His name was 'Veeru'. He died because of kidney failure and I tried to save him but couldnt. I too have guilt trips, but every death has such guilt associated with it. And reading your post, made me remember about my 'Veeru'. I too tried everyting but could not save him. There were times when I felt lazy, when I had to be feeding him lots of water(as part of dialysis). I shout at home whenever the scene of his last breath and when his body went cold in seconds.... comes to my mind. It comes and bites me every time. Maybe he could be cured.Maybe not. I dont know. But I do feel he is around me every time giving me the much needed signals in daily life to cope with depression, and I pray that he is happy wherever he is.

Maybe he(your pet), as a soul, wanted to go. Maybe there was something very wrong with his health which could not have been cured. There is also this point you have to note that veterinary science has not developed the way human medicine has developed. It is better to let go of the negative emotions(in whatever way possible) and remember only the positive moments for your own mental health's sake. Go out and help other animals which are still living, & which need help. That's what I try to do.