Often, the trailer is only used to go to the vet, or drive for longer periods of time. If you patiently practice trailer loading regularly, it isn’t a problem, but sometimes a horse will go in but just plain doesn’t like it in there. Here’s a tip that I’ve used with some horses:
I’m going to sound like Cesar Millan here… it’s all about exercise! Go do something with your horse that gets him or her exhausted. I mean to the point where they are completely tired and sweaty. Ride off at a trot or canter for a few hours. Do something to get their heart racing (from exercise, not fear, of course).
Don’t let them to water, grass or anything during this time. Then, load them in the trailer and give them a huge bucket of water, hay/pellets, and let them rest. So it becomes a place where they can finally take a breather.
When I’m not in the trailer, I have to move my feet constantly and it’s so tiring. In the trailer, I can get to water and rest for a bit.
The trailer = a place of comfort.