If you have the equipment and room, you can connect panels to the trailer and start housing her like that for a while. Make sure all doors are secured open and safe. Thow her food on the floor at the front of the trailer so that she had to at least get her front feet into the trailer, and would be more comfy with all four in. If the trailer has mangers, you could put the food there. Just leave her be, and watch to make sure she is eating. I say to start with the food there because we know she has been comfortable in a trailer before. When no human is involved, and it is all opened up, it may seem less threatening. When she is easily loading on her own to eat for a couple of days.
Go back to working with loading again (but make sure the area is clear of panels. If/when she blows back, step to her side, and drive her into a trotting circle. Keep driving her until she is showing signs of wanting to stop. Again walk up to the trailer to load as normal, and repeat the circleing activity if she blows back again. If she locks up, step to her side, and drive her forward into a circle. If she is being particularily stubborn, get the front feet, or even the head (on a really bad session) in, and then YOU end the session by walking her away. Pen her back up with the trairler and resume feeding that way. Work up to more and more body parts in trailer, always ending by you taking her away from it. For this activity, try to have another horse (calm horse) loaded already. Also, just keep your demeaner calm and relaxed - try to send good vibes.
It sounds like she was freaked out somewhere along the line. :)