Hi, I'm very new here...just signed up in fact. But, I had to respond to your post - 18 hands is a lot of horse to be having trouble loading! Everything here is from my own personal experience. I have two half-drafts, the largest being 17 hands and a little over 1500 lbs. To start, as the others have suggested, make sure the trailer is big enough...open, inviting, and not dark and cramped. I ordered a 7'6" tall trailer that has a total of 11 ft. Length for each horse (8 ft. Body, 3 ft. Head). Don't try this when you're on the way to an appointment with the vet, a horse show, etc. You need to allow as much time as it takes without quitting, whether it is 30 minutes or 6 hours. On a Quarter Horse of mine several years ago, it took 3 hours.
When I trained my two to load, we backed the trailer up to our round pen, which is built out of standard livestock panels. We opened one panel, and backed the trailer up just so it was right inside the round pen, then made sure to close off each side so there was no encouragement to try and escape. It's much easier if you have a helper the first couple of times. I opened the drop down windows AND the full size escape doors, and my husband stepped up on the running board outside the escape door. He had treats within his reach, and I had treats on me as well.
Before you ever try it with a trailer, make sure your horse understands the voice command for "back" or "back up", and this exercise: take a dressage whip (or similar length), stand on your horse's left side, near his shoulder, and ask him to move forward with your left hand on his lead rope well away from his head. Use a clucking sound, or a word (walk, come up, whatever works for you). If he doesn't move forward, away from you, tap him lightly on his hip bone with the whip. If he moves, even just picking up a foot indicating he's going forward, praise him & treat him. If he doesn't move, tap him a little harder. It may take a few times before he gets it, but he will with practice. Once you've mastered the tap on the hip and move forward, you're ready to deal with the awful horse eating monster (trailer).
With treats in your pocket or a fanny pack, lead the horse as close to the rear of the trailer as he is comfortable, without fighting with him. The trick here is pressure and release. When you pick up the lead, you want him to be looking at you wondering where you want him to go. It just takes lots of practice, and they'll get really light. When he stands relaxed (drops his head, licks his lips, or sighs), pat him, praise him, and give him a treat. Then, use your dressage whip to ask him to go forward, even if it's only one step. Each time he indicates a willingness to go closer to the trailer, reward him with praise and a treat. Remember, no rushing and no pulling on him...everything is driven from the rear...the head is only to serve as a "rudder" for direction. Eventually, he will get close enough to the trailer to sniff inside...perhaps even putting his nose to the floor. Depending on whether or not you have a ramp or a step up, the method may vary a little, but it works for either type trailer. If it's a ramp, guide him with your left hand and arm as if you're sending him forward away from you, and cluck. If no movement, tap lightly with the whip, and progressively harder 'til he moves...any movement forward. Praise, treat him, and let him relax. Continue to do this until he finally puts a foot on the ramp or in the trailer. Then immediately ask him to back up. Then, start the whole process over. Don't let him get on the trailer...make him back up every time. Progressively ask him to put more of his body in the trailer (maybe it's two feet). Then back him out, praise him, treat him, etc. Keep doing this over and over, and eventually he will be comfortable enough to put his whole body in. When he does, DO NOT close the butt bar or door behind him, back him off again. Once he's doing that comfortably, put him on and off the trailer several more times (at least 2 or 3). Then stop for the day. Don't "lock" him in the trailer, and don't haul him anywhere.
This is absolutely the best and safest method I have found for loading any size horse, especially a draft or draft cross. I've taught at least 6 horses in this manner. If they learn to enter the trailer without you from behind, you "send" them in. Your helper can pat and treat the horse while you eventually get to the point you can shut the rear door. The helper is also there just in case you get into trouble.
Hope some of what I've said helps...happy trailering!