There is an article in this month's Equus magazine about trailer problems, not just loading. It's not all that informative for someone who knows this already, but they did say that horses can be very comfortable in one kind of trailer, but change one thing, like from ramp to step down , and they go all to pieces.
So, think about what is new and different and work the horse through those things away from the trailer, maybe.?
I hestitate to offer any trailer loading feedback or advice because I have very little experience here, and what I have has been big failures, so I AM that person how is anxious about trailer loading and makes my horse anxious about it!