Are you trying to get her on a stock trailer or enclosed trailer? A lot of horses don’t like to step on ramp load trailers because they feel unstable. Start with a stock trailer, they let the light in so the horse doesn’t feel trapped. Lead her up to it and walk in as if she’ll load. Don’t look back at her because that signals her to stop. If she does stop tie the lead to the trailer and walk away. Let her think about it and make her own decision to load. It may take a half hour or hour but sooner or later she’ll get tired of standing and want to move, and the only way she can move is in the trailer. Be sure to tie high and short so she doesn’t get tangled or go backward. Horses aren’t as dumb as everyone thinks. If you give them a choice they make the right one eventually.
I did this with Jake. He stood there for 10 min, then put one foot in, took it out. Put the other foot in took it out. Another 10 min and he put both feet in, then got out. Another 10 min and my friend I was talking to 50 ft away left, so I walked up behind Jake had the trailer door in one hand so he couldn’t go sideways and just put my hand on his butt and pushed him right in. He was tired of standing. For most horses standing still is the worst punishment, they like to be on the move.