Oh my gosh, that's so hard. I'm not sure if anyone can tell you what to do in this case. I had to euthanize my beloved kitty, Egypt, about 3 years ago after a month of struggling to save him. He had IBD, which wouldn't normally be fatal, but it's possible there was something else going on as well. He became incontinent, was projectile vomiting and had explosive diarrhea. The day I that I knew that he'd been through enough, there was just something in his eyes, a pleading desperation to make it all stop, and I couldn't get him to the vet fast enough. I certainly don't regret trying all that I did for that month, but I also wish his last day was more peaceful and not filled with my frantic hysteria.
I wish you peace and love in this extremely difficult time. Here's a link that talks about quality of life, maybe something in there can help with your decision:
http://72.14.253.104/search?q=cache:...lnk&cd=3&gl=ca