This is actually very effective in controlling your horse's rythm and speed. I have been taught that when the horse is trotting too fast, instead of posting fast and going with the horse's movement, slow down your post and go against the horse's flow. This will unbalance the horse and the horse will usually try to match your posting so that they are balanced again.
I suggest you try this, along with some major chilling sessions with your horse. I am busy doing this with Night Heat at the moment because she is rather crazy in her head at the moment, and for a month I'm going to do some chilling schooling sessions to try and get her whole concentration.
Have you had a look at her feed? The feed can also make a major difference in a horse.