OK so she knows what it is, but just stubborn in knowing she doesn't have to. Get stubborn back. Walk her up to it and when she stops, back her up FAST. Make it more work for her to refuse to load than to just walk in. This might take a bit of time...I once spent over an hour in a training session with one of my 2 year olds working on loading. Be patient, don't get frustrated, and if you DO start getting upset, find a place to stop and come back to it instead of losing your cool and making it a bad experience for her.
Another thing I'd try is a butt rope. Hook it to the center divider, around the horse's back end, and apply pressure at the same time as the halter. When she steps up, release the pressure as her reward and ask again. If she steps BACK into the pressure, I'd pop her on the butt with a crop. Make it count and she'll scoot forward, be prepared to take in the extra slack. Reward for stepping up witht he lack of pressure, and ask again.
It's a LONG process. Easier if they're not scared of the trailer in the first place though. Good luck and keep us posted.