I make the trailer a place the horse wants to be, by making outside the trailer a place of work, and inside the trailer, the place of rest. I also teach my horses to self load, I never lead a horse into a trailer...but that's just my preference.
This means practicing loading on days when you have no intentions of hauling anywhere, and making sure you have a good chunk of time to do so.
If she doesn't know how to lunge, do sending exercises, back well, yield her hips, shoulders, and is soft in the halter on those exercises, you NEED to conquer those first, before even getting to trailer loading, but you can teach those in a matter of days.
Now when you move onto trailer loading, lead her toward the trailer, and before you even get really close to it, start working her; lunge her, both directions, and make her move those feet, don't settle for slow shuffle pace...the more she has to work, the quicker she will catch on. Change directions frequently. Stop at some point, and walk a little closer to the trailer, then before she can really eyeball, and get sticky feet, turn around and walk away, and put her feet back to work for a bit. Then walk back toward the trailer.
Now Do some sending exercises on all sides of the trailer, and get her used to 'squeezing' between you and the trailer...big distance at first, then gradually decrease that distance. Eventually, ask her to stop and rest between you and the trailer...if she does, let her rest for a minute and catch her breath, if she tries to shuffle away, then put her back to work...remember, you are reframing her mind that the trailer is a place of rest, while outside, is a place of work.
When she is comfortable sending between you and the trailer, and stands close to the trailer without moving away, start asking her to send into the trailer, one foot at a time...get one foot in, and then back her out, and put her back to work. Get two feet in, back her out, and then put her back to work, so on, so forth... when you start getting four feet in, start allowing her to stand in there and relaxing...if she backs out on her own...LET HER!!! Just put her butt back to work, and then send her back in! Remember the more you can move her feet, and make her have to catch her breath, the quicker she will figure out what you are teaching her...in this case, where her place of relaxation will be. It might take her 10 minutes to catch on, or it might take an hour the first time, so be prepared, and what ever you do don't give in...she won't pass out on you, but she will remember if you give into her pressure, which will make it harder each time!
I have never had a horse NOT learn how to self load, and it takes that one session for it to really click, as a general rule, and while they might test here and there, you won't ever have the "epic" battle that you will experience the first time through. My current mare could be a brat about loading, and all it takes is lunging once or twice each direction, and I send her to the trailer and she is more than happy to get in, doesn't matter what kind of trailer it is!
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."