Have you tried lunging her? Just lightly, nothing like super fast trot/canter.. just a trot to get her moving around and then try to load. Keep at it as soon enough she'll recognize the trailer means she will get a break. If she puts a foot up then give her a bit of a break to reward the attempt. And keep trying!

Otherwise, make sure your trailer is cleaned out well. I know some horses don't like entering a dirty slippery trailer, or one with strange animal smells.