Sounds like you are just dealing with a bit of a unruly horse who knows that you are unable to command obedience. REally, the only thing that will help you is to get some lessons , prefereably on Sadie. I know you wanted specific advice. But really, you need to have a stronger seat before you can deal with some of the behaviours she may throw out. And being able to get her respect on the ground ,too, will make riding work better.
Sorry I couldn't just offer a few pieces here and there. I am just being honest in saying that you need a more experienced person to give you some help. It'll go so much nicer . And every time you have a ride with Sadie that goes really poorly, where you have her going the way she wants to go, and you almost falling and all, it is actually training her to behave this way the next time.
Hope you can find a way to get some lessons. Good luck.