What you need to try is when the horse freezes and won't move forward is it take up rein slack in either direction with the horse's neck bent and hold it there until the horse takes even only one step in direction of the shortened rein. What happens is the horse moves in the direction of the shortened rein, because it's neck muscles finally say, "Okay". Be prepared for this to take several seconds, minutes or possibly an hour, it all depends on the horse. A word of caution is be prepared for any reponse other than a step in the direction of bend. Give a lot of praise and pats of accomplishment.