I am sorry you are facing a difficult decision, I am sure you will know when the time is right. You can only do your best, at least you gave her a good life. I found this poem, I hope it helps.
If it should be that I grow frail and weak,
and pain should keep me from my sleep,
then you must do what must be done,
for we know this last battle can't be won.
You will be sad, I understand,
but don't let grief then stay your hand,
for this day, more than the rest,
your love and friendship must stand the test.
We've had so many happy years,
what is to come can hold no fears.
Would you want me to suffer? So,
when the time comes, please let me go.
Take me where my needs they'll tend,
only stay with me until the end,
and hold me firm and speak to me,
until my eyes no longer see.
It is a kindness that you do to me,
although my tail it's last has waved,
from pain and suffering I have been saved.
Do not grieve, it should be you,
who must decide this thing to do.
We've been so close, we two these years,
Don't let your heart hold any tears.