My heart aches for you and your little one. You and your Vet are the only ones who can make that decision. I can only say that if it were me, I would let her go. It sounds like she has had a long and wonderful life with you and it is cruel to let them suffer. If she is in pain and losing control of bodily functions then I would say it would be the kinder and most loving thing to do to let her go. My thoughts and prayers are with you, I know how difficult it is.
I have had to let my baby Yorkie go recently and my first Yorkie, Misty Blue was 16 when she passed away. It is never an easy decision to make but you will make the right one when it's time.
God bless
