I think that if she does go through some "terrible twos or threes" how bad and how long will depend on a few things. One is how consistant you are with the "rules". Two is how dominant her personality is.
Just keep in mind that if you're not training the horse, she/he is training you. That helped me when Walka entered his "testing" period. It was mild and very short lived because of consistancy on my part and he really is a sweet heart and good natured boy.