There could be many reasons for your horse’s stubberness, but she does have a reason. The trailer (to her) might look like a dark and scary path. To help this open the side door while loading to lighten the thing up a bit. Or you could also try and make it a bridge to her favorite place… The pattock, grazing area, etc. Make so that she’ll want to go in. You have to have the mind of a horse while trying to fix this problem.