Maybe try riding her with your seat and legs more, it kind of sounds like you are only riding the front end of the horse and thinking too much about the bridle. It really should be the other way around, seat legs and then the reins.
For turning, you body should slightly turn in the direction you are going, it is only a slight shift in weight, remember to sit up look up and around the turn, shoulder slightly in the direction you are turning as well. For stopping, you shouldn't have to lean back at all. Try sinking your weight straight down, kind of like sand dripping down in an hourglass.
Sorry if my description isn't very helpful, I'm not a riding instructor, I'm not too good at explaining things like that, I just know what I need to do but can't explain it! Maybe someone else here will have a better explanation.