Here's some steps to try:
Load her half on, half off. See how long she can stand there.
After you do that, ask her to load 3/4s on--three feet in and one back foot out! If she can stand calmly for that, move on. If she doesn't like that idea, as SOON as she goes to put that third foot in, calmly ask her to back out. A lot of advance/retreat business, here!
Once she's loaded 3/4 in, ask for the last leg--and as soon as she goes to get fully in the trailer, ask her to come out. Do this several times! Then move on to asking her to go one step forward into the trailer, then ask her to back out. If you are always asking her to do something, and never something hard, she shouldn't have the chance to whirl around and run out the door. The trick is to ask for just a LITTLE more then what she wants to give (getting three feet in the trailer instead of all four) and then immediately 'retreating', aka backing her out of the trailer. Thing is, YOU are always asking her to get out... not her deciding to jump out!
Once she can get all the way on, and all the way off, go all the way out and ask her to stand for just one second, maybe two. She should be able to stand still on her own for a good 30 seconds before you start playing with the door... when you move to the door, its just like loading--close it a little bit... then open and back her out. Close it just an inch more... then open and back her out.
It's long and boring and will take several sessions... but there's no reason why this won't work.