Hi. I am so very sorry to read of the complete and utter nightmare that you've just been through. It sounds truly traumatizing. I cannot imagine what it must have been like to watch your sweet boy go through that. Maybe, if you have the means, you may want to seek out a counselor who can work with you through this pain and guilt you're feeling. It may really help.
I also recently had to put my sweet dog to sleep as she was having recurrent bouts of vestibular disease...and back issues...etc. it was very difficult and I have many moments when I feel so full of guilt and like I gave up too soon...all of the "what ifs". In order to help myself through the grief process I ordered a book called "the grief recovery handbook for pet loss" which I found on amazon. I will work through it and hopefully it will help. If not, I will seek counseling.
I think one thing to remember here is that the health of your sweet Georgie was dependent not only on YOU but also on a team.of professionals...some of whom sound as if they didnt really know what they were doing. Believe me, from reading your story I can tel you you did absolutely NOTHING wrong. Nothing. In fact, you did the most loving and humane thing for your sweet friend....you chose to end Georgies suffering, even knowing full well that you would suffer horribly instead...that is the most SELFLESS act of true friendship and love....please try to forgive yourself for anything you are feeling guilty about. Its needless and only makes your grief worse. I read somewhere that guilt is simply our minds attempt to create a different ending to this sad story.
Please take good care of yourself. Seek out support from anyone who you can rely on to give it...professionals included. Be kind to yourself. You sound like a wonderful, caring, most loving person and mother.
You are not alone. Hugs