I had a similar issue with my usually very well behaved ottb mare. Several things came into play. She WILL NOT load into a trailer she thinks is too small for her; she WILL NOT LOAD with someone in the trailer (leading in), and she did not like the ramp.
I ran a lunge line (without chain so they can’t snap it) through escape door and did the tap tap with one hand on the rope, and other hand on the crop. Blocked the other side of rhe ramp with jump standards and a log. She was not allowed to go back, she was allowed to stand calmly in whatever transitional (2 feet in, one foot in, head in).
BEFORE this, I trained the ramp. Any time she put an effort to put a foot on, she got a piece of carrot. Anytime she stood with any foot on, more carrot. Alfalfa cubes in the trailer. She did not like the ramp compared to step up.
Then, once or twice a week, I made her get on and ride around the block, then once a month. People forget that sometimes you have to actively train the trailer (I definitely did), but putting the time in pays off in the long haul.
All in all, took about 3 hours. But now she walks on by herself.