Got through the first half of CA's trailer loading video, and based on his explanations, there are only 2 reasons a horse has trailer issues 1) fear 2) lack of respect. One of those is always present, and sometimes both.
From the explanations, it sounds like a lack of respect for sure in this case. By that I mean you are asking her to back off the trailer, and she is ignoring you. THAT is a lack of respect.
CA bases everything off of his ground work videos. Basically, if your horse doesn't have respect and trust from good ground manners, forget about handling them in the trailer.
So, since this mare's distinct problem seems to be backing, you can try to work on that specifically. Not necessarily with the trailer, but with ground work. Can you back her endlessly around yard just by glaring (with your active body) at her shoulder/chest? (No stick, no pressure on the lead, just your body language) You can work on this for 5-10 minutes every day, for something related to trailer loading training, without actually stressing her out with the trailer. The key idea here is that she becomes SO solid with your cue to ask her to back up that she shouldn't think twice about where she happens to be, if that so happens to be in the trailer, she should still back up and respect your cue.
Easier said than done, of course.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.