^Ugh. One of the hardest things about training horses is that you have to be smarter than they are. I don't know about you, but I find that can be a problem for me (especially with the TBs).
Another thing is to teach your horse TOF before the rein back. Once the 'back up' button is installed, any combination of leg and hand translates to "I want you to back up". Kind of like that saying where everything looks like a nail when all you have is a hammer. Horse gets confused and does the only thing it knows how to do. Total nightmare.