So, my approach is to lead pony to trailer, while holding a dressage whip. Pony is allowed to take his time, sniff the ramp, etc but he cannot look around other than inside the trailer. Head faces the inside of the trailer. Period. He’s allowed to stand as long as he is sniffing the trailer (dude, it smells like you. No one has been in the trailer except you in four years) or otherwise appears to be giving the idea of loading some serious consideration. If he tunes out, I tap the ground with the whip to get a step forward, or I back him up gently a few steps, and then immediately right back toward the ramp. Basically, he has to move his feet after a minute or two of pondering. He used to try to run me over and duck out the side of the trailer, but, he’d smacked on the shoulder for that. Eventually, it is too boring to resist and he saunters on. He is allowed to back off the first time or two but I say “back” so he thinks it was mommy’s idea. He gets comfortable that he can get on and off without being attacked by the ramp or a pack of mountain lions or whatever. But, right back on. We do this enough so he knows loading as a series of steps. Then, he stands there while I do up the butt bar and the ramp. Then I start incrementally sending him the last few steps alone, then we graduate to self loading.
I think the “new” Warwick Schiller (the new, kinder, gentler, post-yoga-mindfulness-therapy version) has some good YouTube’s on low-stress loading.