You just need to spend time working on her whoa. It's best to start in a pen or on the lungeline. Move her forward with pressure then say whoa and stop her. Do that every day for a week in a controlled environment on the ground.
From there, mount her and begin the same transition work. Move her forward with leg pressure then ask her to whoa just like you did from the ground. When she does whoa (because if you put in the effort to this point with enough repetition, she will) you need to give her a minute to stand quietly.
She has to be shown that work is not the only answer. Once she respects you, she will be more content with sitting quietly beneath you. As she learns, it may be hard for her to stand for extensive periods of time. If she does shuffle forward (or backward), circle her forward, back to where she was standing and ask again for your whoa.
All of your work should be done quietly. Easy cues, light pressure. Be sure to ask before you make her do. And expect it to take time. Baby steps. If you expect too much from her too soon, you'll just teach her that rushing IS the answer.