That must be awful for you seeing her keep falling over. Even though she has period of time when you look at her and she seems fine, I guess the falling over will just get more and more frequent.
You are doing what is best for her as her quality of life is only going to get worse. I'm so sorry, it's difficult to get the strength to think that is enough. I know when I've seen Doughnut in pain and taken her to the specialist and if he had said there is nothing more we could do, to see her in pain was just as bad as if I had to have her pts. It's hard for you though as you see good times too.
Lots of hugs. She's had a great life with you and trusts you to do the right thing for her, putting her to sleep isn't breaking her trust.