Jeff, I am sorry to hear about your dog.
I've put down a few "family members" myself and it never gets easier. I'll keep you and your dog in my thoughts. The only advice, and you have already heard it I'm sure, is to look at the quality of life your dog has. I am sure you give her a good home but how does she feel? If she could talk what would she tell you about how she feels?
Isn't it a shame how we are so humane to our pets than we are to people?!?!