If the divider comes out, take it out and give him a bigger space to step into.
I had this problem last year and fixed it the next day within 10 minutes. This is what worked for me.
Park trailer in open area where you can work the horse. Put the horse on a long line, 14' or so will work. Near the trailer entrance, yield the horse's hind end in both directions. He needs to move his hind end away from you when you step toward it, not just walk around in little circles. If he's not crossing over with his hind legs, he's blowing you off. Do not get too close to his back legs, Stay up by his shoulders so, if and when you insist on him doing the exercise correctly with a flick of the lead rope, and he kicks out at you, you won't get kicked. If he does kick out, immediately back him up into last Tuesday. Flick the lead rope at him, tap him in the chest, whatever you need to do but back him up. Once you think you've made your point. Quit only after he is yielding willingly from both sides.
Next, again near the trailer, send him between you and a fence, a wall or even the side of the trailer. The goal again is for him to walk calmly between you and the solid object. First do it from a good 10' away, but then gradually decrease the size of the space he is to go through.
Now take him back to the trailer, yield his hind quarters just long enough to establish who's calling the shots about where he is to move is feet and then lead him into the trailer. Come to a halt, praise and then either back him out of let him turn around and come out forward if you've taken the divider out. Repeat until he goes in willingly many times over.
Next apply the sending exercise. Send him between you and the opening, and then send him in. Expect him to go in just as he went between you and the solid object. Forget that the trailer is the trailer and he will to.
If an anytime he becomes beligerent, resume the leg yielding exercise. Once he realizes that his tantrums will only result in more work, he'll get the idea and load. Do not get angry or aggressive with him at any point in this exercise. Do be patient but also assertive. He must understand is to go where you say, but that no harm will come to him for doing so.
You just have to see your distance...you don't have to like it.