Tune into your horse. Why is she feeling anxious about going into this trailer? Is there a specific reason, do you think, or is it more of the classic being-flighty-cuz-of-the-creepy-boogey-man-looking-box-on-wheels-she's-trying-to-lock-me-in? ;) Do all you can to make her comfortable...and this starts with you. Do not act confident, dominant, or like the leader......BE confident, BE dominant, and BE the leader. So do not attempt training her loading when you are in a pissed off mood, for example. A mistake a lot of people do is act differently than how they are feeling--well, horses KNOW, so that's not going to fly. :3
When your energy is aligned, tune into her energy. There are several ways you can do this....
1) Do you know her color? Colors help me as a mantra. Now what exactly is this crazy person talking to you about? :P Imagine a shield of your life energy coming out from you inner core and surrounding you peacefully. What color is it?
exactly what color is it? Next time you are with your horse, put your hands on her; imagine your energy engulfing and mingling with hers. Now: What color do you see? This is the color of the shield made when you combine your energy with your horses. This will be different for everyone, because it is the combination of YOUR SPIRIT and HER'S. Really a beautiful thing. ♥ Anyways, recalling this color will calm your energy, and with your bond, it should calm her as well.
2) Recall a peaceful scene--it can be real or made up--that is pleasurable for you as well as your horse. In this case, you want it to relax your energy. For example, my most common energy-aligning scene is imagining myself soaking the spring sun in a wild field, Horse grazing beside me, with a creek flowing nearby....... Practice this image. What some people do is create a "button" that recalls their memory/emotion. For example, pushing your right thumb onto your left palm, or even something like squeezing your arm (so that it only takes one hand). When you have your "button" and your scene, practice "pushing your button" and immediately being engulfed in the scene throughout your day--while getting the mail, while at a tack store, while watching a movie.....you get the idea. ;]
Okay...as I probably have proved already, I could go on for hours on this...and I didn't even get to the real practical part yet.

So basically, what the gist of this is, is instead of trying to convince her to move her butt and feel uncomfortable about it, convince her to move her mindset and feel safe and accomplished. ^^
If you have any Q's let me know...again, I really am forcing myself to stop writing!! (if you can't tell :[ lol)
~The Golden Filly!~