I'm sure I'm going to get lifted out of the heavens for this but...
This is a situation where you do not have the time to be nice. You are going to tell this horse she is getting onto the box, end of story. No nicey nicey treats and baby talk. Try and get a person either side, and a person to lift her fronts up to the box. Make it as welcoming as possible, take the divider out of needed and make sure you are doing this in the day light, and that the box is bright. She's been in a box before (that's how she got there afterall) so don't let her take the mick. Be firm but fair, and make sure you are well stocked up on "bottled patience"
For one incredibly difficult loader we once used a large roundbale of straw and rolled it up behind him and used that to push - he kicked the heck out of, did no damage to himself and no damage to us. The next day we had to load him, he walked right on in.
Stop for a minute, open your mind, learn. You may not agree with what I say, I may not agree with what you say but we will both learn something new.