I'm working a horse who was the opposite. On the ground he was great, turned out he moved when asked, but once in the saddle he refused to move, and had a tendancy to rear up, as his previous rider only took him on trail, and he learned that he got beligerant, and she'd end the ride and take him home. He's a huge 6 year old halter Quarter Horse, who spent his first few years doing halter shows all across CA and Arizona. I just kept asking and pushing and as soon as he moved forward, even a step, I'd reward him, and tell him he was a good boy, and it took a few weeks, but now, he actually doesn't want to stop. He loves to be worked, and gets very excited when I show up. Just find something that works to get her to move even a step, and take baby steps, bond with her, get to know her, and soon she should start listening better. Seems like maybe whoever first rode her did very little arena work, and so now she thinks that she can get away with doing nothing, and especially since you are new, that she can take advantage.