The moment she moves her hind end away from you, pull her head towards you on the left side of her mouth. Pull her around so that her head is facing you, and with your left arm wave her over to your right. If you are carrying a whip, wave the whip at her shoulder until she moves over. Once she moves over to the point where you could walk her forwards to be mounted, you stop and praise by patting her face. Ask her to walk forward one step with the left rein, but be careful not to wave her forward with your right. Just cluck at her, or maintain a gentle tug. The second you feel her take a step, stop and pat her. Repeat until she is far enough forward for you to mount. If she moves her haunches away again, keep repeating.
What this does is the closer she is to you, there is a place of peace. When she moves away from you, the area she is moving into creates pressure, and so she will chose to remain square with the mounting block. :)
In riding, a horse's energy is like a river- guided by the banks but not stopped by them.