Trailer issues are so frustrating. I'm not familiar with your story, Nu, so I'm sorry if these come off at "know it all" advice on things you tried first! Have you been able to try several different types of trailers? I worked with one horse who preferred a fully-enclosed side-by-side over the wide open stock type trailer people typically recommend.
Do you have someone who can take your mare and just walk her into the trailer as if they expect she'll load? Perhaps your mare is feeling your frustration since you are so close to the situation; your tension could be telling her she should be worried about entering the trailer.
She's a OTTB, 7 yrs old, I got her mid December from the Humane Society of North Texas. She loaded right up with no problem in their 6 horse slant. It took 3 trips to get her, 2 with large stock trailers, and last one with 2 horse. They had to ACE her to get her to load, and it still took a couple hrs. since then, I've tried everything, Clinton Anderson method included, Nothing. I backed our stock trailer up to one of the tiers in the pasture to see if it was stepping up into it that bothered her, nope. But, she'll back into the trailer no problem! But, how many places can I go that I will have a place to put the trailer where it is level with the ground. After weeks of backing her in, feeding her her nightly grain in the trailer, using butt rope to pull her in, she finally out of the blue, walked in like it was nothing. but, she won't stay in if I try to get out and leave her in, unless I go out the front side door. She pulls back and has a fit if tied in the trailer, so don't want to do that while she's having problems with it. after a week of her just walking in, tried it without the grain, as stated above, you know the rest! . Maybe I'll back her in, drive her around the block (which is big in the country) do that daily for awhile, show her it's not scary