Excercises you can do:
1) Lope circles. When you feel her speed up, turn into the fence. Lope in that direction, and repeat every time she doesn't rate. Simple but it works good
2) Lope circles again, but instead of turning into the fence, stop and back uptoo much speed. When you feel . Not only slows her down, but develops a more rounded lope. Again, simple but effective
3) Lope circles until she speeds up, then spin to the inside of your circle. In other words, if you're loping to the right, spin to the right. Then as soon as you make 1 spin, lope off again to the right
4) Also another thing you can do is just practice making smaller circles.
All the tricks I use involve circling. Don't know why, it just works for me