Rearing while loading in trailer.

Hi all! Would love to hear some suggestions, as I've exhausted a lot of my knowledge and resources. ............ Problem: A horse who has trailered before, now once she's half on rears backward violently and hits her head on the top edge of the door. She's done it twice now severely, once requiring 12 stitches. I'm very worried about her doing further damage to herself, one because I love her, and two because I want to show her and obviously do not want her to have a mangled face..................... What I've tried: 1) Asking her to go in, if she refuses, I go work her away from the trailer for several minutes to show her the trailer is a rest. Can do this for hours and hours on end with no progress. 2) Working her around the trailer, then showing her inside is the rest. 3) Asking for only a step at a time and giving reward when she does so. This works until she's half in, then she explodes and hits herself or just narrowly avoids hitting herself if I'm lucky............... She will happily go half in, but as soon as she gets her hind feet about to go in, she very violently rears backward and hits herself. If she were just rushing backward I could work through that, but it's twice now she's hit her head badly, as she's throwing it up very high and hitting it as she almost falls over backward coming out. She hasn't seemed to 'learn' not to hit herself, either...................... The trailer is a nearly new, bright, open, 3 horse slant that is fully opened for her to go in. It is not a 'scary trailer'........... I want to be able to work through this issue, but it's so dangerous what she's doing that I can't allow this behaviour to keep happening or she's going to do something very serious/permanent to herself................. Would love to hear some suggestions! Sorry for bad formatting, not sure how to change it. I'm new. Cheers.