**WARNING LONG POST** (sorry)
Listen, you will undoubtedly get a million and one responses on the right way to trailer your horse. I've been in your situation and I assure you I understand that nothing is more frustrating than loading an unwilling horse. Add onto that the frustration of everyone and their mother coming up to you with their suggestions which usually tend to freak out your horse even more due to unknown handlers testing their limits with brooms, lounge lines, blindfolds and other antics. Im not saying that they don't work for some, but this is what I did for my horse:
I backed the trailer into a naturally made corner that gave me about he same space as 1/2 a round pen to work in. (If you don't have fencing or landmarks that make this possible, create your own circle with temp fencing or wheelbarrows as long as the horse doesn't refuse violently in which case these things could hurt him/her if they chose to bolt) I turned on some music that I now keeps me calm, because patience is of the essence and I'm known to run low on such things. Then I grabbed my horse and got her in the "semi circle" and continually asked for her to walk onto the trailer. If she balked, standing firm and not making an attempt to listen, I would spend some time by the trailer but not necessarily loading onto it, reminding her to only back, walk forward, or stand on my cues. To do this I just ask, with as little pressure as possible for her to back, whoa, walk forward, whoa. When she does it with 0 resistance and is essentially following my movements rather than any pressure on the halter/lead, then we return to the trailer. AT NO TIME does she get to just stand and hang out. Once she takes a few steps on the ramp/into the trailer, she is praised and asked to back out, even if she is willing to move forward further. Eventually you will find you have to "let" your horse come all the way on to the trailer. Day 1 took 3 hours. Day 2 took 2 hrs. Day 3 took half and hour. Day four I as able to load her perfectly fine on the first try and then throw the leadline over her neck and allow her to self load several times before letting her be done for the day. Please note, we never went anywhere. It was simply a loading lesson, a few days later she self loaded and we went on a trail ride, no big deal.
A few things to be aware of
1. I never use food or treats. Cats and dogs are carnivorous/omnivorous and therefore must chase food naturally, therefore food is a reward. Grass never ran from a horse, therefore it is a poor reward as they never have to work for it naturally. Not to mention my horse is not food motivated anyway. However, many people find success in feeding the horses daily feed on the trailer to desensitize them.
2. I do these lessons often to keep my horse on her toes. If I don't for a few months, sometimes I'm treated to a frustratingly long session of loading (usually with me running late)
3. The goal is to have your horse WANT to load. Pulling, bribing and kicking them up the ramp is a quick fix and will only make it that much more difficult when its time to load again.
Hope this helps! I understand your pain for sure! Also, maybe look into a farrier and vet that travel to you-- those visits arent always positive for a horse and if they're trailered off the farm often enough for them, then the trailer seems very ominous to them, making it a sure bet they wont be happy/willing to load.
This is brilliant, however it needs to be noted if you don't do groundwork with your horse, that needs to be the basics, so she puts her feet where you want her to.
As for using a whip, some people may or may not agree with me. If you ride with a whip, or carry a whip, the horse knows what its for, its used for impulsion, especially from the hind end. My dad's mare was a nightmare, rearing up, bolting, planting.. everything. Nightmare loader. I took her, carried a long dressage whip, walked on, when she stopped, I flicked my wrist and tapped a hind leg, which she then moved forward off. Now, it she loads fine.