Be sure that when you work with her in general that she doesn't get her way and that you are the head of the herd....She may just be asserting herself and it may be a simple training issue. On the other hand, check that your trailer is not a miserable ride for her. Perhaps she remembers being tossed about or perhaps the trailer is loud and rattles a lot. She might not appreciate that either.

I'm not sure this will help, but it's a thought.

Good luck

Sometimes my easy to load horse, won't load... I always have the same tool ready when it happens though.  I use a lounge line and loop it through the trailer window, hook it to his halter and give a little pull with the left hand on the end and he will always walk right in that way.  He put up huge fights in the past and I discovered that it was the only way to get him in when I was trailering alone.  Now he almost wants that lounge line hooked on... sometimes i hook it on and he does not know it isn't through the window but walks right in anyway. 

I can say that once I decided to load and unload until he would walk in with ease.  I did that and he walked in with ease when he wanted to, but still has days when he does not want to load.  I really don't know why he does that but do have that lounge line ready, before I go catch him up, just to make sure I can immediately load him if he refuses.  This might not be much help to you but thought I'd share my experience.

Sounds like your horse has a respect issue and also does not view the trailer as a happy place to be. There are several clincians that have good videos on trailer loading. I prefer Clinton Anderson method for teaching respect and trailer loading. I also use a reward system for young horses and teaching old ones that have never backed out. There are some other posts on HTW that go over what other folks have done too - just do a search.

Also, make sure there is not a lot of rattling gates or floor boards that is giving her a bad ride. And make slow starts and stops when hauling.

Follow PonyTammys advise .Respect,move her feet gain respect.

If she is not loading RIGHT AWAY then she still is not trained!  Keep it up.  She is doing this because she can!   BTW, this is a leading problem not a trailering problem.  She should virtually lead anywhere you lead her - whether it is on a trailer or through water!