Is she really nervous around it or just a bit stubborn? I have never had a problem with my pony, but a friend of mine has a very nervous and VERY stubborn arab who refuses to do anything if she is the slightest bit scared. What she did was park the trailer just outside the gate near the barnyard, and put some hay and oats in there. It took them about 2 hours, but they got her to walk in twice with no spazzes! They led her to the door and praised her every time she got close. Eventually she would stick her neck in and sniff the floor, then she would put one foot in, then the other, and after 2 hours her whole body was in. They left her in there for a while and let her eat and gave her lots of praise, then they walked her off and brought her in again.
If that doesn't work, you could try monty roberts method. Some friends of mine got there extremely stubborn percherons in the trailer using this. Heres a video of how he does it: