Cindy, please do not beat yourself up! You have gone above and beyond for Gavin that few owners would have done! You are to be commended for the love, devotion and caring you have given your boy! You have even gone the extra mile to help others who might encounter a similar diagnosis and with all your detailed posts of his diagnosis and all the treatments.
I do admit, I also felt the same way in the latter stages of Mr. Chance's decline. I looked back and was really angered when I would bring him to the vet hoping to catch any issues early to be proactive about his care. To the extent that one of the vets just said to me... "he is an OLD Dog". To me, she was just writing him off! I just won't go back to a vet that has that little compassion as to say that to a person who has such a deep bond with their pet. We all hope we could do more and correct anything that might go wrong. We do the best we can. That is all anyone can expect. Hang in there.