(((Beth)))
I'm so sorry. Our family has been there, too. It really is a quality of life thing. When you do take her in, consider being there with her when the vet administers the shot. We all went in with our Boo (she was nine and had an antibiotic-resistant bacterial infection that, after three years, was just worse and worse and left her in constant pain) and just petted her and loved her and told her it would be okay as the medicine took effect. It was important for us to be there together as a family and face it together.
It's really hard to know when it's time if your dog isn't in actual pain. Ours was, and it was affecting her quality of life in a way we loved her too much to let her suffer through.
Good luck making your decision.
Roxy
Getting in touch with my inner try-athlete.