First Problem, try and get a stock trailer. I know MANY horses who will not load into a horse trailer and even having a rear tack makes it worse. We have a stock trailer because our one horse does not like and travel well in a horse trailer.
Our mare Sassy when we got her was deathly afraid of trailers. When I mean deathly, the moment she would see one or hear one (rattling) she would go sycotic. You couldn't get her 30 feet near the trailer. It took us at least 4 people to load her as her owner wanted to just get her in. We backed the trailer up into the barn, and we had 1-2 people trying to pull her in, and we had a butt rope on her. She would eventually give into the pressure and walk in. We once fought with her for an hour in pouring rain to get her in. We did try the drugging trick with her and she still had a mind. It took a butt rope to get her in.
One day we had someone's bigger trailer, ours was only a 6' high trailer, and we had someones 6'5. I asked to go work with her and I was able to. I took her out and let her go sycotic and eventually got her to the trailer and I walked her round and round that trailer and let her sniff every square inch until she didn't want to smell it again. I walked her infront of the door closed/open. Her owner had told me that "You will never get her in, you might get her close but she will never get in that trailer without force". She eventually got comfortable with being around the trailer, I let her smell the inside, look in, etc... I would be in the trailer and give a firm tug and hold it. She usually would resort to backing so I would walk her around the trailer sniffing, etc.. She made one step in and I backed her out and praised her. In 20 minutes she walked into the trailer. This is a 16hh Thoroughbred mare I was talking about who is built like a tank. She had some trailer trauma.
You could try that method... or
I talked to a trainer and they said about lunging them infront of the trailer and then asking them to go in. If they take a step in then praise and let them stand there for a bit. Then back them out and lunging them by the trailer, attempt again and let them rest when they have a foot in.
Now.. If you want to go with force methods.. then I will give you ideas.
Now that same thoroughbred we hadn't trailered in over a year. She wasn't scared of the trailer when we tried loading her a few weeks ago, she just didn't want to load. When we left the vet she was still sedated from having her teeth done and she was pretty out of it before we walked out. The moment she saw the trailer you could tell her mind just switched. It took 5 people to load her. It was already dark and 7pm and the ground was icy so I couldn't lunge her. The vets wanted to go home too. They took a lariat and tied it to one side of the trailer and got two people on the otherside and put it on her hindquarters, and had someone whipping the ground behind her. Did she try kicking? Yes. Did it still take us a while to get her in? Yes. I have also seen at that vet they would take a lariat and put it around the hindquarters. And then pulled the lariat and they could control the hindquarters. Our stud hates loading into a trailer. Likes to rear. But if we take a rope and tie it to one side of the trailer and put it behind him and if it just touches his legs he will walk right in but if we don't its a war.
So those are a few ideas. But I would really recommend getting a stock trailer and opening all doors. Your horse might be claustrophobic.