I don't have a trailer, I have to borrow my coach's. Given that THAT would be the best way to retrain, I am looking for other ways to control the way my horse exits. This is what I've been doing.
Get him comfortable with moving between and backing up in narrow spaces...using barrels in various places. Get him backing up straight on the ground, and asking/varying the number of steps forward or backward. Using a labyrinth (aka tellington method) and asking for forward/backward steps along the long sides. Asking to place one or two feet over a log, pole, walking forward and or back. In short, trying to control his feet and help his mind. He does back up nicely lowering his head, which I like. And he doesn't seem get to frazzled, even if he hits the pole and makes it move around.
I wish my coach didn't have a straight haul, but til the time comes when she's able to provide access to her trailer, I am doing this stuff above. He does load better now....it is just the unloading. The longer you 'fiddle' with un clipping him and opening the door, the worse his anxiety.