When I first read your post, I wanted to ask, the same question as palogal
, I would work with this horse on better response to my seat. Speed her up with your seat, slow her down, reins would be my last choose when asking this horse to slow down. The reaction you are getting is due to the fact your horse is fighting the pressure on her face, reaction rather then response is a big sight that you might be asking for to much to fast. Think about how you ask your horse to stop. If the first thing you think is reins then that is where I would start. Work on think slow then seat then last reins.