This is a really common problem with an awful lot of horses. We have tried the backing on the ground and found that most of these horses back just fine on the ground. It is only a trailer thing.
We have tried the one foot in and back out. Then two feet in and back out. [And about that time, many of them will just barge in and charge all the way to the front of the trailer.]
We have tried a chain over a horse's nose and most of them just bang their heads repeatedly on the roof.
When I had a two horse, I tried leaving one in the trailer, parked in the arena. The next morning, he was still in there. looked like a gutted snake, but was still there.
That is when I recruited a helper, put a long rope on each side running them out the back of the trailer and the two of us just pulled him out. Took about 2 or 3 minutes. Gave him a drink, loaded him back up and did it again. The time it only took a little pull. Third time I could back him out by myself.
So, this is what we have done ever since. It has never failed and no horse has ever been hurt. We did it 2 weeks ago with a big, 1250# 10 year old App gelding that I bought at the last local horse sale. [The night I bought him, I hauled him home loose, not knowing how he tied. He met me at the back of trailer so he unloaded the way he was used to. I know better than to start something I can no longer finish.]
The next day we tied him solid and found him OK with that. We sacked him out and he was OK with everything. So we loaded him to see how he tied and backed out of a trailer. He not only would not back out of the stock trailer, but would try to bulldoze right over you when you got him to back 1 or 2 steps inside the trailer. He would plow right over you and charge all the way back to the front.
My arthritis and degenerative back is so bad that I cannot pull of push at all any more, so husband got a guy to help him and they pulled him out with two 20 foot ropes. He tried so hard to go forward that they had to take turns taking a wrap around the side bars on the trailer and take him back a little at a time on each side until they got him to the back. He finally stepped out. I took about 10 minutes. He tried a lot harder to go forward than most.
Husband reloaded him 3 or 4 times until he backed out very nicely. He took him on a trail ride a few days later and he backed out of the trailer without a problem. He had to go into the trailer to back him out, but he never hesitated.
All of our other horses back out without anyone even getting into the trailer to get them. We untie each one, one at a time, from the left side of the trailer, tell them to "Back up!" and they carefully back out one step at a time. One of our trailers hauls 10 head and they all load without anyone leading them and they all back out one at a time the same way. The App will, too, after a few times of hauling him.