So sorry for you, because it hurts and you get attached. But it also sounds like you are sensible enough to know that it isn't fair for her to suffer. You did your very best, and she at least had some love and caring in her short life.
It is true that animals sometimes sense that their off spring are not healthy and will abandon them... Seems cruel, but it is nature's way of not prolonging a life that cannot survive. I commend your caring, and agree that it is time to let her go, so she doesn't suffer anymore.