My rat wasn't as old as your poor little girl, so when he had a very icky eye wich he couldn't see with anymore,I let the vet remove it.
First we tried for several weeks on antibiotic but his eye kept getting bigger and bigger.
I asked the vet to remove it because my beautifull guy was in pain because of the swelling.
Twice! he sent me home and told my the rat was not in pain :?
I demanded for metacam to show he's wrong and that worked!
In a few days on the metacam,Lucky got much more energetic en playfull again. (he was only 7 months old at the time)
At last the vet amputated the sick eye.
I can feel the pain you feel,because you don't have that option
Good luck and al the best for you and your little girly
raying: