If she is swinging her hind end around, then you are pressing too close to it. Move the target of your pressure further forward, even as far as her nose. When asking a horse to move its' sholder over, if pressure on the shoulder doesn't work, pressure on the face will! Horses will always move their face away from pressure. YOu can use a tapping motion with your fingertips, or the head of the whip and if she doesn't move by tapping the air near her head, then move in and actually tap briskly on her jaw or nosebone. She will move, and when she swinges her head over, you put pressure on ther shoulder, becasue it will notwbe more available to you. When the shoulder steps over, stop all pressure and give praise.
It's always harder for a hrose to step sideways or backwards than forwards. It's hard for the horse to step sideways without any forward motion at all , and if she is learning, then it can be mentally challenging, too. It gets easier, though, doesn't it?