I do it a tad differently. Start your horse on 20 meter circle. If she is going faster then your desired speed cut your circle down to 10 meter (or smaller if you have to!). Make sure you sit down, slow your hips, pick up slightly on your reins and tell her "slowwwww". As soon as she slows take all the pressure off and let her trot a few times around the that small circle. If she keeps the trot move out slightly larger, and larger. When she breaks it you bring her down into that tight circle until she slows it down. Eventually you will be able to get her out on the rail and if she starts trotting to quickly, again, small circle and back on the rail.
I hope this made some type of sense! If not I'll clarify.
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