What do you do when your horse wont stop backing up?

What I do is, when my horse backs up when i want her to stop or go forward I MAKE her back up about half way accross the arena. Everytime she starts to un-wantedly back up I make sure she does, but for how long and when I want her to. Once you have done this, your horse will willingly go forward!! :) Repeate this everytime your horse backs up on her own, and after a while she/he will relyse that everytime she wants to back up and does that your going to make her back up more than she wants to and she will quit! Good Luck