I have used Kitten Val's method also and it works well as they see the trailer as a place to rest.
With horses with traumatic trailer experiences, I like to be sure that you can control each foot independently on the ground before we ever approach the trailer. I have a platform that they all start on as a foundation for trailering. One foot on, one foot off, so on & so forth, all when you ask. I always add a verbal "Load" with this training. I can point any of mine towards the trailer, toss the rope over their necks and say "load" and they put themselves in. When you get to the point of her putting a foot in the trailer, back her off before she gets a chance to do it herself, make it your choice. Keep at it until you can put, 1 foot in and out, both fronts in & out, etc. Don't rush her or try to force her in as she is coming from a place of fear. When she does get in, make it a big party, love her up big time and back her back out. Eventually the bells will go off and she will think "oh yeah, going in there means getting loved on".
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.