Everyone has given you good advice! So I'll just add one thing that helps me.. think slow and easy. I tend to over think and analyze everything; so what I do is find a saying or count or even sing; anything to stop myself from over thinking situations. When a horse is going too fast, I think slow and steady, and say it nice and slow. Or if Im trying to keep a rythm I will count out loud (for canter) 1-2-3-1-2-3 to the beat of their foot falls in the pace I want. What this does is slows your body down, and you actually give signals to your horse to slow down. It can be hard, we are taught from the beginning to go with the horse's movement; but with this you are actually going somewhat against it.
Its the same thing in the trot; you want your horse to go slower, post slower!
Hope I explained this okay, Im not quite awake yet
if you need anything explained differently or cleared up, don't hesitate to ask!