Yes, starting with the attitude raises your adrenaline, which sends the horse into fight or flight. Put all of that out of your mind, and think positively that the horse is going into the trailer, and will remain there calmly.
The AAEP had a seminar several years ago on the research that using light doses of drugs is an effective training tool. My vet at the time had attended this convention and seminar. I used it several times with great results to haul mares who have never been trailered, and no chance to train. There may not have been a problem.....but rather than set up a fight, we avoided it, and the mares returned home in the trailer unaided, just walked right in and rode.