My very first horse, an Arabian gelding, didn't like to load into 2 horse, straight load trailers (I later discovered he was fine with stock and slant trailers that were more open).
Anyway, luckily the old owners gave me an important tip, use a butt-rope and he would load right up. It's not painful, not frightening, it just puts pressure on the horse until he feels he has no place to go but forward into the trailer.
So anyway, I didn't own a trailer and we were moving some years later and I paid this guy to haul my horse to our new home.
I told him the horse was funny about loading but if we put a butt-rope on him he would load right up. So the guy was cocky like "I can teach any horse to load without a but rope. Give me a lunge whip." Okay, gave him a lunge whip where he proceeded to go after the gelding's rear heels.
The horse took to kicking at the whip and he gave ME a dirty look. I said "let's just put a butt-rope on him." The guy was a little peeved at that point (or maybe he just couldn't bring himself to use a butt-rope?) because we ended up locking arms behind the horse and he loaded right up.
Anyway, that is just a story from my first-horse days. Also, a butt-rope is a good thing to try. Not all horses may respond to it, but it's not scary or painful. And it can really work on some horses. So it's worth a try.
It's like the horse knew that the gig was up with the butt-rope and cut out all the shenanigans, to the point that locking arms behind him made load right up.