I have a 6 year old mare that has a history of sulling up at the gate. For a time, a couple years - I trained her not to do this - but she has revered back to her old ways. She seems to do it more-so at the show-gate than at the farm. We are moving in a couple months and I am concerned that she'll start doing it again. It seems that she does it more in new environments. How would I go about keeping her from sulling up in new places?