Awe; this has to be a tough time. I can sympathize with you; awhile ago, we were close to getting rid of one of our horses b/c of horse-on-horse aggression issues but things cooled down and they finally settled on a pecking order and things are fine now. But I know the feeling. I also know that you know what is best; it sounds like she might just be a one-dog-house type of girl. My cat is like this; he loves humans but viciously attacks dogs and other cats so we are not getting anymore until after he passes on.
I wish you luck on whatever road you have to choose.