How are her teeth and have you checked her gums? When she picks up speed, play her game and go along with it, don't try to slow her. But, when she wants to slow down, that's when you change the game. Keep her going. She will be puffing and you will feel her begin to lug under you. Keep her going. When you want her to slow, relax your body and at the same time let out a deep breath. I promise you she will slow. I'm wondering if you are guilty of holding your breath in anticipation of her picking up speed. If you do then you are telling her something is amiss and horses run from that. It's helped them survive for thousands of years. Before you try playing her game, try humming a dull boring tune. This will keep your breathing which helps relax her.