I hear about and see and have dealt with many horses that have trouble loading in a trailer and I have found that it usually is much more of a foundational issue and really doesn’t have much to do with the actual trailer.
In order for you to get your horse to get inside this scary small box it’s imperative that you have his respect and trust and the ability to move his feet in a forward manor.
If you can’t lunge your horse both directions, stop him and turn to change directions without moving your own feet much, “send” him forward through a space (another post to follow on “sending”) like a small gate opening or between yourself and a fence, and move his shoulders and front feet forward and away from you, then you can’t very well expect him to lead or “send” into a trailer.
So, if you find yourself having trouble with the trailer, go back to the basics and get the trust and respect you should have first. Get your horse soft and responsive in his ground work and manners, with respect for your personal space and build the desire to want to be with you because that’s where the pressure goes away. Then watch your partner do anything you ask him to do.