It's very sad but sometimes cats just don't get on. They are solitary beast by nature and as they mature their personalities comes out. It may just be that she would be much happier as an only cat. It's is a heartbreaking decision for a loving owner to have to make but if the cats genuinely don't get on, it is kinder for the cat for her to be rehomed. Living with inter-cat agression causes stress all round.
Hope you can find her somewhere lovely.