I also agree with what jwells and TBbuttermilk. However, what my trainer had to finally do, and this was with his slantload stepdown trailer, was to put the but bar up. When Walka started to back out and then went to rush, his butt hit the bar and he immediately came forward. My trainer would then step him back one step at a time, if Walka rushed again, he hit the bar again.
Finally after 2 - 3 times of this, Walka figured out to listen to the trainer and did not rush. After , the trainer took the bar down, and Walka did not rush, as he was focusing on the trainers cues.
He doesn't rush backwards in my trailer, two horse straight load with a ramp, mostly because of the ramp. Plus I spent hours practicing unloading while practicing loading.
Good luck to you, a horse ,as you know that that rushes out of a trailer is very dangerous to everyone around it as well as itself. It is thinking in flight mode, not really thinking.
Be the kind of woman that when your feet hit the floor each morning the devil says, "Oh crap, she's up!".