There were a few weeks there where my horse would NOT come near the trailer. She would not take one step up. She fought everything we tried to do, and some of the stuff we tried was: have a rope around the nerve on the back of her head or a chain over her nose, and use pressure/release (this didn't work that well). We tried smacking her with a whip and the lead rope etc, making outside seem as scary as possible and inside the trailer calm and relaxing, tried bribing her with treats (please don't call me out on this one, I was extremely desperate and I knew even then it's not the right thing to do) and what always seemed to work was putting a lunge rope around her butt and hauling her in. Still, I always hated the lunge rope way, and it was stressful and hard work for everyone involved.
So basically what I did was start at the very beginning. Not many people have stooks of grain, but we did, so I'd put a sheave or two in the bottom of the trailer and simply sit or stand in there while she ate, letting her be relaxed and within a day or two she was starting to step up onto the trailer. Eventually she'd come all the way on. I'd praise her, then quickly back her off.
After a week or so, it evolved into her willingly coming onto the trailer, and when she didn't, I'd repeatedly back her up, bring her forwards, back her up, bring her forwards, until she got so bored that she just said: "Ok, I'll get on the trailer." After a few weeks of doing that, she started to refuse again, so I had to do something different.
In the final "stage" of it, to get her to walk on I'd send her out of my space, and back her up and do all that, but wouldn't let her near me. Then I'd walk up to her, pat her and tell her to follow me. With that, I'd walk away, head up looking forwards, taking confident steps and never looked back at her and when I walked onto the trailer she followed without hesitation. When she did hesitate or refuse, I'd just send her away again and do the same thing. What also worked was when she wouldn't step up I'd step off and rush her-make her dance and spin and move her feet and hassle her, then would walk over to the trailer where I was instantly calm, and show her that if she doesn't follow me onto the trailer I'm going to harass her, and she's going to have to move and work.
At the point where you're at, just spend lots of time around the trailer. Feed her in there, make yourself comfortable and get her used to just looking at it-make sure she associates the trailer with relaxation and calmness (is that a word? Calmness?). The trailer we were using at that time was small, dark and extremely old and in less than perfect condition, and by the end of several weeks' work she had no problem in it.
When your horse does get to the point where she'll put even one foot in, praise her PROFUSELY, give her treats and big pats, and then I would always back her out immediately. I guess you could encourage her to step all the way up, but I never wanted to push it too far. It just depends on your horse and what you both are comfortable with, I guess. One foot leads to two, and two leads to her back feet coming up as well. If she feels confident and safe in knowing that she can back up and get out whenever she wants (well, at the start, anyways) she's probably going to be a lot more likely to be comfortable in there.