I am so sorry for your loss. I just had to have my cat put down because he had cancer. It was just a few weeks ago and the pain is still there. I think about what I could have done differently also. I should have taken him to the vet sooner is my biggest regret. Maybe the cancer would not have spread yet and he could have been saved (he was in surgery to get his leg with the cancer amputated, but they found it had spread and the best thing for him was to put him down). It sounds like you tried very hard to help your cat. Plus, it does not sound like he would have eaten regardless of the type of food on his plate. I can tell you loved your cat very much. I try to look at it as my Jack is not suffering anymore. I can look at pictures of him and smile without tears. Allow yourself to mourn without the guilt and remember the joy and love you two shared. I know it's not easy, but for them we have to be strong so they can rest peacefully knowing we are ok. Hang in there.