Since there is no one right way or one wrong way to train a horse, you may want to change the method you are using to train your horse. You have several different ways to choose from. If you are not having any luck with one method you can always try another.
Here is another method of training your horse to go into a trailer.
The first thing that you need is communication between you and your horse. It is necessary for the horse to understand what it is you want him to do and you must make it easy for the horse to understand what you want him to do.
Once the horse gets good at moving both directions then it is time to hook a lead rope to his halter and lunge him left and right. Just as an example, when you point left you want the horse to go to your left, and when you point right you want the horse to go to your right.
The end result will be this. You are holding the lead rope while facing your horse. While you are holding the lead rope in your right hand, you will hook the lead rope in your left thumb. Lift the rope to the where the horse can see it and point to the left.
Because your horse knows what to do, he will immediately go to your left. After a circle or two you will switch hands and using the same motions will make your horse go to your right.
After your horse has mastered that, send him or lunge him through a gate opening, barn door opening, etc. You will also want to use smaller spaces to lunge him through to give him the opportunity to deal with his natural claustrophobia. You can accomplish this by lunging him between you and a fence. As he goes back and forth from left to right make the space between you and the fence smaller. Take care you don’t get stepped on.
Once he has mastered that, you can take him by the trailer. Open the door to the trailer and allow him a chance to sniff it. Then you will step back and while facing your horse, you will lunge him to the left and to the right in front of the trailer as he passes back and forth by the opening.
After all the pointing and sending your horse knows what you want. You have communicated to him when you point to the left he is to move to the left and so on.
The next step will be to make him face the trailer. With his head facing into the trailer and you standing on his left side you will then point and send him into the trailer. Some horses will simply jump right in the trailer where others may take more work.
If your horse will not go into the trailer after pointing and persuading, take him away from the trailer and make him work. Now you are transmitting the idea that being in the trailer means he gets to relax. Being outside the trailer means he has to work hard. Sooner or later he will catch on.
After you get him into the trailer, you need to pet him and let him relax. You want to let him know how well he did, so talk to him.