Sounds like you're being too nice. If a horse went in that far and refused to move, I'd be getting a bit more aggressive with loading her regardless of whether she'd flip out or not. She's playing a game and she is getting her way over and over again. Make it stop.
Someone on here worked the snot out of the horse and the only rest the horse saw was if it went into the trailer. Eventually, the horse understood that the trailer meant rest and the only way to get rest was to get on the trailer.
I've done the "rope butt" thing before. Someone else mentioned looping the rope through the trailer windows and pulling her while someone else used the butt rope. I like this idea. Pull/push, the second she takes a step in, release. Give her a rest for a could minutes, maybe a handfull of grain and then do it again. Inch by inch, you'll head into the trailer.
Have you tried twitching? If your vet won't drug her for the ride, find another vet. Seriously, you're letting this horse get away with murder.
At this point, being "nice" isn't the answer. You're afraid she'll flip out...well let her flip out and find out what the consequences are for flipping out.
The "food in the trailer" idea is another nice approach. Eventually you are going to have to meet this head on and force her into the trailer. Put some food in the trailer with the set-up you have right now, but put it all the way at the front of the trailer so she'd at least have to go in half way to go get it. Come in behind her with lungelines (you'll need a couple people for this) to act like a fence. Pressure her in. Use a lunge whip. Her ONLY CHOICE is to go forward. She CAN NOT go back. There is a FENCE there now. Don't be scared to whip her little butt. Its time to get this done.
You can also move the roundpen panels up behind her when she goes in to eat. She can not back out. She can only go forward. Use a lunge whip to reinforce this.
And those who were seen dancing were thought to be insane by those who could not hear the music