It's amazing what kind of memory these long ears have. I've read that if a mule has a bad experience it can take years, if ever, to get them over it.
Our littl yearling decided she didn't like the halter. It was going to eat her. It would take both of us in the corral with her to eventually get a halter on her. Once it was on, she was fine. I started all over with her by taking the halter in the corral with me and laying it on the ground. She LOVES to have her fanny scratched so she will back into you to let you scratch. I would slowly scratch up to her withers and then she would run. If I knelt down, she would walk head first to me. It is now to the point, if I'm kneeling down I can put the halter on. We've had her since she was 3 months old. No one here has ever done anything harsh to her...... who knows