That's the hardest appointment in the world to keep but I always think about how many wonderful years you had with them and when you got them you knew it was only for a while. And if she's truly miserable, what a kindness it is to give them peace.
And if you believe in this sort of thing, Petzi Sis went to an animal psychic after she put down her Binkey and the psychic said Binkey told her she was so grateful to be let go and that she still came and sat by Petzi Sis.