Today I worked with three different horses that had all issues with trailer loading... the most important message I could convey was the conversation with the horse should initially focus on refining the tools needed in order to present the trailer loading itself.
So many holes can appear if folks took the time to assess without intention, but rather with awareness. They would start to recognize how early the horse will show his concern or lack of understanding. This allows the human to help refine the communication while their horse can "still hear them" rather than waiting until the horse's peak stress levels and then hoping to get a change in them.
As with many scenarios, I find trailer loading "trauma" whether psychological or physical, causes so much anticipation that tends to limit future improvement.
By learning to separate each scenario, the handler can offer more effective and influential support in "real time" to the horse.