With every horse I work with on trailering, we always start slow, as I’m sure you have. But, that includes teaching them to back up as they learn to get in.
So, what I would suggest is starting again. First of course one foot, whoa, step back. Done for that lesson. Next, two feet, whoa, step back. You get the idea, it’s just as important for them to learn how to get out as it is for them to learn to get in! It doesn’t matter if they are learning with this technique using a ramp or step up because it is so slow. If it were a ramp, it would be the same thing except the first step is onto the ramp. I always want them to stop, think and relax with treats at each forward and backward movement. I want them to go slow so that they don’t feel the need to hurry up – that’s when the wrecks can happen. As I look at your responses above, it’s pretty much the same as Ann2ponies has above, but I’ve just added the relaxation, waiting and treats. Good Luck!