Mostly trailer loading is not a big problem with handling horses. But if you have a horse that does not want to go on the trailer, this is a difficult and stressful situation. You’ve already tried everything, outsiders come up with the strangest ideas or your horse was put on the trailer by someone else (in a too harsh way for you).
People don’t realize what kind of tension this gives to a horse, even without those strange methods. I think it is important that the horse learns to walk into the trailer in a familiar way, remains there quietly and walks out in a relaxed way.
How many sessions do you need?
That differs per horse. Some horses are relaxed enough with half an hour and then just go to competition / class / vet. Some horses need an hour, and some horses benefit from 1 or 2 repeat days.