More often than not, the temptation of feed cannot overcome the fear of something tragic that has happened inside the trailer.
Take your horse on a long lead-rope or a lounge line. Open the trailer door, and take something like a long stick with a plastic bag on the end or a rope on the end or something similar, even the end of the lead rope or lounge line works well. Begin to circle your horse in the entrance of the trailer, as near it as possible. Make sure he/she stays at a fast trot. Stop your horse by stepping in front of he/she at the entrance of the trailer. If he/she is willing to sniff the trailer, look inside, step inside, whatever, just don't do anything. Don't pet the horse, just let him/her relax completely. Once the horse looks or steps away from the trailer, circle them again, 2-3 times, and stop them again. Eventually, your horse will learn that they can relax at the trailer, and work away from it, and will look forward to relaxation near the trailer. This takes more time with some horses than with others. The point of this is that you are letting the horse overcome it's fears and make the choice, not forcing it. Ideally, your horse will circle, see the trailer entrance, and step inside eagerly. Be patient if your horse is very resistant and keep working on it. Once your horse is willing to be inside, spend meal-times and grooming-time in there and once your horse is very used to the trailer, just tie he/she up in there and just let them relax. Good luck!