I forgot to add that bribing the horse, confident or unconfident, will not work. It won't work for confident horses because they just don't care and they aren't going in the trailer b/c that's what they know you want, and it won't work for unconfident horses because they have safety issues so food is not even on their radar. Bribing a horse at any time is not good.
I have to respectfully disagree. I have bribed a horse to catch it, I have bribed a horse onto a trailer and bribed that same horse to get out of the trailer, I have bribed a horse to get in and out of a pasture, on and on. Not that it's the way I'd prefer. But it does work for a temporary fix.
Sometimes you just need to get said horse from point a to point b and then you work on the fine tuning later.
I prefer to trailer train a horse when you have no where to go. When you can take your time and reward each effort of the horse until it's understanding and loading properly. Like I said before, if the OP can borrow the trailer, patience and practice will solve the issue.
Trailer loading, like many things in horse training, can be such an emotional topic! Not to sound wishy-washy or wordsmithing, but I think the bottom line is that how it's done depends very much on the horse AND the situation.
As I see it, the goal is for your horse to load and unload by him/herself whenever you want, regardless of the trailer or what is going on around him/her, without expecting a reward (i.e. This is part of the horse's 'job').
If you reward, that's fine, as a 'bonus' for the horse.
Just some experiences of mine, for whatever they are worth...
Our 14 yr old seasoned, ex-penner mare will walk in and back out by herself without a treat or reward (though we do reward her). All you have to do is walk her to the trailer and she'll load herself. Remember, though, that she is experienced, has been to countless shows, and has done this hundreds of times. As good as she is, you do have to watch when unloading because if she has too much room, she will want to try and turn instead of backing out. I expect it's just normal for a horse to prefer to go out frontwise instead of backward, especially in a step up trailer.
Our 6 yr old mare had only been in a large stock trailer a couple times before we started working her with our 2 horse stock/combo. After about an hour, she would load and unload at home every time without problem using a hay reward for loading (she loves to eat). Trailer her out to a friend's place, and she loaded fine after an afternoon of riding.
Second time out, loaded fine at home, but after the ride, there was a 4H show at the ranch with lots of horses in/around the arena, and it was obvious that she just wanted to stay and watch. Practiced more loading/unloading at home later and she did fine.
Third time out, (remember, only her 3rd time actually trailered somewhere)loaded fine at home, but after the ride (with a larger group), there were a lot of folks helping people load, and she would go to the back of the trailer and just stand, not wanting to go in. After a few minutes, I stepped back, scratched my head, and saw that she had her head down, taking a nap....she was happy and relaxed, enjoying all the attention from people trying to help...I'm sure she was thinking, this is great!, why get it?
Our 4 yr old mare had never been in a trailer with a divider either. I don't have an urgent need to trailer her anywhere yet, so I'm just taking my time working with her, without and with the divider in, and she's gaining confidence.
I guess this is just a long way of saying that, IMHO, it depends on the horse, experience, age, what motivates it, the trailer, the environment/situation....a lot more things than just the trailer.
...and...if it's 7pm after a day of riding and it's going to rain, you don't want to spend all night trying to coax your horse into your trailer to go home. As Dumas'_Grrrl said, you just need to 'go', and if all else fails, I've found that a surprise swat on the butt will get those hind legs up and in (though you have to be prepared so you or your horse doesn't get hurt). There is that point where I think the horse believes he/she has 'out smarted' you, and you have to make being in the trailer look like a better choice than not going in.
Sorry for the long post. I hope my experiences will help someone.