U need to work slowly at it u have to open every thing in the trailer and put hay at the front of the trailer and just let it take its time and don't make it go in but encourage it with treats or hay so they might come in but do that every day maybe twice a day and after a bit u will see a difference I did it once and it worked.
For a horse that is not afraid, just resisting because they have associated the trailer with work, have a helper stand on each side of the horse with a dressage whip to gently encourage the horse forward and prevent then from swinging sideways. If they plan to their feet and refuse to go forward, make them back up and keep backing up until they want to stop (if you have it, up a hill is perfect, they tire out quicker). Then, because they are desperate to go forward, steer them straight for the trailer and march purposefully to it, not allowing the horse to waver. Repeat until he want to go on and give a huge reward once he does go on.
Patience and gentle persistence! I had a mare who was very difficult to load (she had quite a few reasons contributing to this issue) and with lots of work I was able to train her to load without issue.
I would plan as many trailer training sessions as you can do (I usually did twice a week) where you dedicate a half an hour or whatever you can spare just slowly getting the horse used to the trailer. I had her get used to being near the trailer, then putting one foot on the ramp, and so on until they are comfortable being all the way on.
I cannot stress enough that you cannot rush the horse or get impatient, that will erase everything you've done. If you are rushing the horse, they can 100% feel that energy and will not go in.
As Isabela suggested, food is a great way to get a food motivated horse to get more comfortable, but for a horse that is frightened, you will not be able to tempt the horse on. I would sometimes put a flake of hay just inside the trailer so my mare had to stand on the ramp to eat it, but didn't feel pressured.
Once you've done tons of there training sessions, and you're having success easily loading every single time, and you sign up for a show, be sure to leave tons of time for the horse to load, because again, if you're rushing they can feel that.
Patience is the only way to deal with a scares horse. I hope all this helps!!!