Beautifulbay, if it was me (and there wasnt much traffic) i'd try to pivot the horse to where its butt is facing the direction you wanted to go, then I would let it back away! LOL. I'd give the horse a few opportunities to turn around and walk like a normal horse, and if they went to backing up again, i'd turn around and repeat. A good cue for you to do this is when the backing up seems to slow down.
But im passive aggressive.. so don't listen to me if my method sounds unethical, lol.
But heck.. it gets the job done.