I am going to draw more than one naysayer here, but I just do not believe that horses have or show fears and phobias like people do or like people think they do.
I think they can and do 'panic' after really bad experiences and then they just plain decide they are going to say "No!" to anything similar.
I have re-habed a number of horses that have been in trailer wrecks. My old stallion I owned for many years became mine because he was in a bad trailer wreck and crippled. He had to be cut out of an over-tuned trailer and spent a year in a sling at the University of Mich where they saved his life. His left leg was almost severed at the knee with the joint capsule and all the ligaments severed or badly damaged.
The other horses that I have re-trained that had been in over-turned trailers but were not hurt as badly, all got over it and hauled regularly. One was a roping horse. After I re-trained him for his owner, they went right back to hauling him to ropings 2 or more times a week.
I have re-habed TB race horses that flipped in the starting gate, got up-side-down in it and had to be drug out by several men. Needles to say, starting gates, trailers and doctoring stocks were not on these horses' plates.
I had a horse arrive up-side-down in a 2 horse trailer at my training stable. We tied his body to a post and drove the trailer out from under him. He was in self-destruct mode and was going to kill himself if we did not get him out quickly. Two women were all that was anywhere around -- the trailer/ horse owner and myself. You do what you have to do.
I found out that you can over-think and over-compensate for a horse's past quite easily. I have found it best to establish a very strong respect bond and get these damaged horses VERY, VERY obedient. When you do, they will come to depend entirely on you and will not fight you. When you do, they are nervous when put into the situation they fear, but they get over it.
I have found that using a 'draw-halter' or a 'lip-string' to teach a horse to step forward when told will get ALL of them in the trailer or starting gate or stocks or other place that they fear. This is the ultimate 'pressure and release' mechanism that one can use to train a horse to do what it really does not want to do.
There are two main things that have to be adhered to:
1) You have to do everything with them in a calm and quiet manner. You have to make them uncomfortable when they do not step forward, but you never want to go far enough to actually hurt them. You absolutely HAVE to keep them in a 'thinking mode'. Hurting them will put them on the defense and make them react. A reacting horse shuts down all thinking and learning.
2) You have to recognize when a horse is 'thinking and responding' and when a horse is going into a reactive and non-thinking mode.
Absolutely no learning takes place when a horse is reacting and not responding.
So, the instant a horse has been pushed too far or too hard and it starts to blow up
resist in any way
you need to back off and let the horse get back into a thinking mode. This is probably the most important thing I try to teach people when I attempt to teach them to 'read' a horse.
The secret to it working is to put them in and back them out BEFORE they 'blow up'. Do this many times - keeping them in for a longer period of time each time and they will get over it.
My old stallion, Doty's Socks Jr, learned to get back into a trailer this way and was hauled in a 2 horse from Indiana to Western Colorado when I bought him.
The horse in this post is 'reactive' and the owner is fighting the horse which makes it even more reactive.
I do not have time right now to put together an entire post that goes into minute detail on how to get a horse quietly into a trailer with this method. I have said quite a bit about it before, but I will try to put the exact details of how it works. I will say that in the last 45+ years that I have used this method, it has never failed to get a horse in a trailer -- usually within an hour and it will load repeatedly in any trailer -- day or night -- anywhere. It will go in the trailer for no reason other than you told it to and you have taught it that what you say is what it must do. It is not an 'or else' thing. It is a thing where the horse just wants to do it 'your way' -- like Burger King.