OP, go over to the other current trailer loading thread for a good instructional video of Clinton Anderson loading a horse. (I don't endorse CA altogether, but the proof is in the pudding here). You'll see that he takes the horse through the panic of the trailer by just asking the horse to walk by it, till the horse loses panicky fixation that he's going to get pressured into trailer, then CA sends the horse back, doesn't allow horse to fully enter, even though horse would have, to instill in horse the understanding that backing out is safe. CA also says to practice with all sorts of things on the ground, as we've been saying. It's pretty much all there. :)
Notice also how he 100% gives the horse the time to check out the trailer, every time the horse wants to, & says how important that is for the horse!!