There is an old saying that there are too many good horses around to waste time on one that isn't. It sounds like Stella has about every bad vice known to horse and from her pics she doesn't appear to be that special. That said I learned how to teach horses to load over 40 years ago and the method has never failed. However, loading and riding in the trailer are two different things. Horses that refuse to load for whatever reason can usually be taught to load. Horses that turn violent once they are in the trailer are another matter. In my opinion that vice is too dangerous to the horse and the owner to try to fix. Here is how I teach loading and it is really pretty simple. First of all, I will not go into a horse trailer with a horse. Being in a confined space with a horse is the most dangerous position one can put themselves in if something unexpected happens.
Imagine the opening of a horse trailer being like the entrance to a spooky tunnel to a horse. If the horse is already apprehensive and the person is standing in front of the horse then they have become an obstacle in front of or inside the spooky entrance to the tunnel that the horse doesn't want to enter. It is much easier to drive an uneducated horse into a trailer than it is to lead it in. This is the basis of the natural horsemanship methods used by trainers such as Clinton Anderson and Monte Roberts. That said, I tried Clinton's method on one of my horses and it didn't work. Oh, I had the horse loading alright but it didn't stick. A month later I had to retrain the horse. It was probably me and not the method.
The most common factor with horses that are difficult to load is owner cowardice. The horse and the owner are both apprehensive to begin with. It takes an hour to load the horse prior to leaving for the horse show. As soon as the horse is in the trailer the back door is slammed shut. "Whew! Glad that's over," the owner says as he/she wipes the nervous sweat from their brow, jumps in the truck and takes off without ever taking the time to teach the horse to load.
Horses learn by repetition. If the trainer has a system for loading the horse and then uses it to load and unload the horse 20 or thirty times in one session then the horse will probably be good for life. The method used is not as important as the repetition and there is always more than one way to accomplish the same thing.
Here is my system. I can usually teach a horse to load in an hour. Park the trailer next to the barn so the horse can only evade you by running around one side of the trailer or backwards. For the purpose of this discussion we are using a two horse straight load, step up trailer. Take a 30 foot long soft rope and run one end of it through the eye in the manger and back out to the other end of the rope. Put some sweet feed or alfalfa in the manger. Using a bowline knot tie one end of the rope to the halter that is on the horse. (If yo haven't learned to tie a bowline you shouldn't be messing around with horses
) I use a nylon web halter because I do not want the horse to experience any discomfort on its head during the training. Ok, to review, One end of the rope is attached to the halter. The other goes into the trailer, through the eye and back out to the trainer's hand.
The horse is led as close as it will walk to the entrance of the trailer while the slack is removed from the rope. As soon as the horse refuses to move forward the trainer moves behind the horse at about a 45 degree angle from the hind quarters, far enough away not to get kicked. The barn is on the other side of the horse and you are loading into the side closest to the barn. At this point the horse will probably run backwards when it feels tension on the rope. Don't try to keep it from backing or you will suffer a rope burn. You can stop the horse with the rope when it is 8 or 10 feet from the trailer. It is a horse's natural instinct to flee away from the object it is afraid of. We remove that fear through repetition. The horse may try to avoid approaching the trailer by going around the side of it (the side you are on) and will have to be repositioned. Little by little the horse is encouraged to move forward by putting pressure on the rope. As it takes a step the pressure is released and then reapplied. The object is to get the head inside the trailer. Every time it puts its head inside give the horse a little slack on the rope and allow it to relax for a moment and then back it six or eight feet and repeat the process until the horse will calmly approach the trailer and put its head inside. The trainer is still at the rear or to the side of the horse. Finally, it may be necessary to tap the horse on the hind quarters with a buggy whip while giving the command to "load up" with some tension on the rope to get it to step up into the trailer with its front feet. Back it out and repeat but don't over do it. Now encourage the horse to step in with its hind feet by tapping its hind quarter with a buggy whip or training stick. The horse will probably back out a few times before it steps in with its hind feet. As soon as it goes in all the way allow it to eat for a minute or two and then back it out of the trailer and immediately repeat the process tapping with the whip if necessary. Repeat this 20 or 30 times until the horse does not resist. Do not attempt to close the rear door until the horse will enter the trailer willingly and stay there until asked to back out. Now you are ready to load the horse without the rope. Put a lead on the halter and remove the rope. It is useful to have an assistant to man (or woman) the whip. Lead the horse in a circle up to the trailer as it approaches the step tell it to "load up" as the assistant taps it's hind quarters with the whip. As the horse enters the trailer throw the lead across its back. Soon the horse will walk right in on voice command without being tapped. The last step is to load the horse solo without anyone behind it. Carry a dressage whip in the off hand, lead in a circle to the trailer, give the command to load up. Reach around your back and tap the horse on the flank with the dressage whip if necessary. If the horse balks have your assistant as a presence behind the horse clucking until it get the idea. It won't be long before the horse is hopping right in. The object is to have the horse load calmly without charging into the trailer and banging into the manger. It should also unload calmly. Overuse of a whip or stick will cause the horse to leap into the trailer.
While it is true that you are applying a pulling pressure forward on the horse since you are also behind it's center line you are driving it forward from you. Everything you are doing is encouraging the horse to move forward and it is not impeded. A word of caution. Some horses can become violent when you are saying, "My will, not yours," especially spoiled horses. They may kick out, rear, run backwards or to the side. I have had them rear and come down on the top of the trailer with their front feet. I can not remember having any failure. Persistence and confidence have always prevailed but this method is not for the novice.
One last observation. The greatest joy for me is the bonding that takes place between me and the horse I am training. I have never had a good horse that has not become more affectionate and cooperative as the training progresses. I would not keep a horse that has problems that keep it from bonding and becoming my best friend.