I know the OP has solved her trouble, but I thought I'd post this for the benefit of others.
The mare in my avatar was the same as the OP's horse. She loaded fine when I bought her, but would not back out of the trailer when I got her to my pasture. She showed the desire to turn, so rather than fight her at the time, I let her try to turn around. It was a two-horse Shoop trailer without a divider, but it was too narrow for her to make the turn. She ended up pretty much falling out sideways and got a couple minor scrapes. It was a mistake on my part not to fix the problem at the time, but I just didn't have the time to mess with it.
When I decided to fix the problem, I got a two-horse trailer with a divider, so she could not turn around. I did this by myself, but I recommend a helper.
I fed the lead rope through to the rear on the side against the wall, so I could pull from the rear to coax her to back. Then I stood at the rear with a second rope and captured a rear hoof at the pasturn. I lifted the hoof up and to the rear with the rope, staying well clear to the side, so as not to get kicked. She kicked and tried to free the hoof, but I just held the pressure until she relaxed about it. I lifted the hoof outside the trailer and let it down. It took me several minutes to get her to let it go all the way to the ground. I worked like that with each hoof, and within about an hour had both hooves outside the trailer on the ground. All this time, I also put pressure on the lead from time to time to attempt to coax her to back a step. She didn't give much up to this point.
At that point I pulled on the lead rope hard enough to get her to back. It took quite a pull, but was not harsh. She resisted, but by that time she was tired and I was able to get a couple steps back from her. She was about 3/4 out of the trailer when she reared and was about to leap back into the trailer. All of a sudden it was like, "Oh! I'm out!" and she came on out. Overall, it took me about two hours. I think I could have done it in about half that time, but I wasted a lot of time initially just trying to pull her out with the lead rope.
She was edgy about backing out two or three more times, but after that she has been fine.
I now have a 4-horse stock trailer that was converted by a previous owner to a 2-horse with a camper. It has no divider and is wide enough for her to turn around in, but I still require her to back out. I want her to keep the habit of backing carefully out of the trailer.
I think one thing I accomplished with this method, is that she backs out carefully, rather than charging backwards as I have seen some horses do. I used no brute force, no punishment, just gentle, but firm and constant pressure. I used a regular flat-braid nylon halter and a long, strong 5/8" lead rope, and a second short rope for the hoof control.
Last edited by thenrie; 01-06-2013 at 10:47 PM.