I have such overwhelming guilt right now. How do you deal with it?

I keep thinking what if I did things differently? Should I have put him to sleep earlier? I also keep asking myself if I should have had him hospitalized earlier thinking it might have avoided any suffering on his part. With all the vet visits added up (and there were quite a few!) I probably could have afforded for him to stay a few nights at the hospital to receive better treatment. I know I probably shouldn't even think that way but it weighs heavily on my mind.