Oh god... I feel for you. It would be a hard call for me. I love my dogs like most people love their children (I'll fight you if you have a problem with it too), but they don't live forever.
I talked myself into a $3300 knee surgery for an otherwise healthy and happy 4-yo lab mix - no regrets. Recently paid about $1500 for the same thing for a 9-yo beagle, who's already running like a crazy puppy again. I don't second-guess those calls. But the older they get, the more of a dilemma it becomes. Now that they're both past age 10, we're more likely to just try to keep them happy and comfortable till the end, rather than stress ourselves prolonging the agony.
I hope that whatever time you have left with her is good time. Give her lots of love and treats. :)
I think someone's cutting onions in my office... stupid onions...