The toughest thing is bracing with my abdominals. I kept telling myself, brace with the abdominals! Abdominals! At the same time my instructor would be saying, brace with your back! Your back! So I kept getting mixed up and trying to brace correctly. It IS true, if I brace with my back the horse just slows down and stops. Then I tap her with the whip to make her move forward and I just feel bad because she's getting mixed signals.
You're paying her to teach you to ride, and so far she's the only one out of everyone 'here' who's seen you ride, so I would take her advise first :) She seems like a good instructor, I like her already for avoiding training 'aids' like forks or side reins with a new rider.
Never be afraid to ask questions with your instructor, that's the fastest way to learn.