Welcome to the Forum, nuisance!
Make the right thing easy and the wrong thing hard - make the trailer a great place to be, and make the area outside the trailer a place where she's going to work. I use NH-style lunging to accomplish this and have had success with several problem loaders.
Open the trailer up as much as possible - the bigger and brighter it looks, the less scary and claustrophobic it is for your mare. Start by just leading her up to the trailer; don't look at her, just walk purposefully forward like you know that she's just going to step right aboard. Up and In.
If she doesn't step on, allow her to get as close as she wants (assuming that she truly is frightened of the experience and not just "saying no") and sniff the floor, ramp, etc. As long as her attention is on the trailer and she's obviously thinking it through, let her be to relax and think. When her attention wanders away from the trailer, push her out onto a circle and lunge her. Make sure you have enough line that she has room to move and the circle isn't too small - I use about a 15-20 foot line. Do lots of transitions on that circle, both changes of gait and of direction. The name of the game is to get her feet moving with energy.
When she's responsive and thinking about what you're asking, go back to the trailer - don't stop between the circle and the trailer, just smoothly take that energy toward the trailer. If she hops on, great! If not, allow her to rest as close to the trailer as she'll get for as long as she’s thinking about the trailer. When she chooses to leave the trailer, push her out onto the circle again.
When she’s resting, make sure that your body language is neutral. Allow her to make the choice to either load or “flee,” and let her make the connection between rest and thinking about the trailer, and between that choice to leave and getting more work. She needs to learn how to load mentally as well as physically. I’ve used grain to get horses into trailers before, but I don’t like to – the horse doesn’t learn anything. The only time I go for grain is when I must get the horse in the box to go home, yesterday.
One final thought – loading is only an extension of other leading activities. If your horse is truly proficient at leading, you can take her anywhere on foot without hesitation or being dragged, and you can control her body from the ground, trailer loading shouldn’t be a problem. Never underestimate the value of going away from the trailer and polishing on basic leadline skills.