In all honesty, why even talk with her about leaving. Just leave. YOu can call her, but you dont' have talk WITH her. Just say, "I have decided to move to another barn". If she asks why, then engage if you so choose. Otherwise, just tell you won't be in for the next lesson and find another barn.
If that trainer is abusive to the horses, and you participate by riding them when they are lame, or too young, or in the manner which is overly harsh as per her instructions, then YOU are being abusive, too. Is that what you want?