I gotta say, I can empathize with your predicament OP. My mom and I just spent the over a month retraining our mare to load quietly and stand quietly on the trailer. We had her loading without hesitation, standing with the trailer all closed up quiet as could be.
Then we moved the trailer about 10 feet at an absolute crawl -- and she lost her mind. She wouldn't stop kicking and screaming for other horses even after the trailer was sitting there motionless for 20 minutes. Now we're back almost to square one, where she won't load all the way on the trailer and won't stand there anywhere near long enough to close the trailer up. It makes me want to rip my hair out.
The sad thing is, we have no choice but to move her in another week. Our boarding contract ends and she has to be moved to another facility. We thought we would be proactive by starting on her trailer training over a month in advance... but at this rate we'll be fortunate just to get her on the trailer, slam the doors closed behind her... and haul her temper tantrum meltdowning self all the way to the new barn. >_<