My horse doesn't stop when loping/trotting?

Practice LOTS of transitions from walk to trot and trot to walk then trot to canter and canter to trot. Don't let her get too forward at the canter and don't pull on her all at once, keep a feel and start pulling and if she gets out of control you can always try a pulley rein. but thats mostly for emergencies/when you absolutely can't slow her down. And NEVER let her eat gr when you're riding her. its fun but starts really bad habits:/