Have you checked to be sure there aren't any bees, wasps or hornets or fire ants in it? Is it real hot inside the trailer? Is it wide enough for your horse with the divider in place? Three feet wide is usually sufficient unless this is a draft type horse. You still might want to take the divider out to give her more room until she gets very use to the trailer. Some horses just won't accept a narrow trailer stall as some slant loads have a narrow stall when closing the divider. 

Are you giving her plenty of lead rope so she can drop her head down to see where her feet are going when she starts to load? Many folks pull on a horse when loading them & raise the horse's head up where they can't see where to put their feet when stepping in.

Is it dark inside your trailer(painted dark) even in the day time? If so, can you turn a dome light on?

Is your trailer kinda high up off the ground? If so, try backing it up to a small hill, so the step up isn't quite so high.

If all of the above is in order, then try putting a lasso rope like ropers use on her butt like you have roped her butt. Kinda like one would do when teaching a foal to first lead. And when she balks give her a little tug on it. It will tighten around her butt & usually kinda goose her forward. Once she starts to move forward, quit pulling on it. Some horses will do fine with this method, but if she's truly scared of the trailer, she might start kicking at the rope & feel of it on her butt. So void that plan...

Pictures of your trailer, inside & outside where she loads & stands would greatly help for other ideas...

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