See if you can borrow a trailer with a ramp, that seems to help with the backing out part at first. If she loads great, but just doesn't want to come out you can practice with a trailer with a ramp. Walk her part way up the ramp and then stop. Let her stand there for a minute and then ask her to back all the way off the ramp. As she relaxes start to go a step or two farther into the trailer and repeat until you have her all the way in the trailer, let her stand for a couple minutes and then back her all the way off. This may take several days of work if she is that afraid to back out. She can't see directly behind her, so a horse must really trust to blindly back out of a trailer. Just remember to start easy and then build slowly on that so that she doesn't get scared, you're trying to boost her confidence in the situation.
That's all I can really offer to help, I hope it will help you. Good Luck.