My upper level dressage mare decided to do that one time so as she was backing I turned her (still backing) and she backed up past the "bogey man" object! A couple of times doing that and you can send them forward.
Make certain when you ride forward the rider has their head turned AWAY from "bogey man" object and outside leg (opposite side from bad object) back to prevent horse from escaping in that direction (alittle is OK at first). Horse CAN feel the weight difference (of the head) and believes scarey object is no longer of concern for rider.
(like it scared you to begin with