Carl,
I am so sorry that your family has to deal with such a tough issue. I can only imagine the pain you are dealing with in making these decisions.
Personally, I would put my pet down if it was in constant pain, but that can be hard to tell sometimes. You are the best judge as to if your pup is happy. Keep in mind that your pup has had a great life with your family, and has no regrets about her life. Do what you think is right, when you think it is right, and no one can fault you. Certainly everyone here will support you in such a tough decision!
Your pup loves you unconditionally, and would thank you if she could for the great life you have given her.
Peace be with you, my brother....
Randy