I am not a horse expert AT ALL..but I feel your pain and feel obligated to give you any ideas that might help. Trailer loading has been my curse. Of course I am the common denominator here so I take full responsibility as even horses that were known to load would not for me. But that is the past!!
I have a gelding that is as stubborn as they get. At first would not load (I need them to self load as I am usually by myself to do this). I got him to walk on but as soon as I moved a muscle to raise the butt bar he would fly out. So two things I would suggest. The first is taking from Clinton Anderson is that if you have a horse that backs out faster than you like, the minute all four feet hit the ground make them work, circles, lots of direction change so they have to think, if not circles walk with them on a long line (as long as they are trotting), send them over jumps, anything until they get a bit winded then load them and let them rest. Don't even think of closing them up until they look relaxed in the trailer. Then ask them to back off, if they fly off again....same thing. Outside means work, inside means peace. Unfortunately, at the time he was in pretty good shape so it took a lot to make him
tired.
The other thing that actually worked better for my guy (NH trainer's idea, not mine) was that if he backed off before I asked him to, I got after him with ugly sounds and popped him on the butt with the popper on my lead rope (not too hard, mind you just enough to get the message across) then sent him right back on. After about 2 times all I had to do was make the ugly noise when he took one step back and he would pop right back in. Now when we get to the trail he stands and waits for me to lower the butt bar and ask him to get off. He is not a kicker so I knew I could be a bit tough with him. And I have a ramp which made this easier and not as scary. With a step down, maybe you could build a platform (railroad ties or something sturdy) so that at first the step is not too large.
He is still not perfect and will still throw a tantrum every now and then to get on, but he is always on in no more than 5 minutes and once on stands politely while I close him up from behind first, and then secure his front.
TRUST ME!!! If I can do this, I am sure it is easily fixable and should be a piece of cake for you!
