My heart and thoughts go out to you. It's never an easy decision to make, but only you know about her quality of life.
The last paragraph was written for a dog...but....
http://www.geocities.com/animalupdates/DogsPlea.html And, my friend, when I am very old,
and I no longer enjoy good heath, hearing and sight,
do not make heroic efforts to keep me going.
I am not having any fun.
Please see that my trusting life is taken gently.
I shall leave this earth knowing with the last breath
I draw that my fate was always safest in your hands.