Wow, I really feel for you. I am sort of going through the same thing with my cat, Bootsie, who will be 19 next month.
The only advice I can give you is to do what feels right in your heart. Only you and your partner know the quality of life she is having, whether the bad days outnumber the good days, and whether she is in pain or not. I think pain would, for me, be one of the biggest deciding factors.
It sounds like you have given her a wonderful life and you love her dearly. These decisions are always so hard to make and I truly empathize with you.
owned by 2 Newfies, a cocker spaniel and 3 cats.