She's funny with the bit though, we have a myler snaffle, the type that rotates the side you pull, not sure the name. She seems the most comfortable with this one, all the others she can't even stand just sitting in her mouth, she fusses for hours. This one she's relaxed with. She gives fairly quickly to the right but to the left she just chews and chews and gapes her mouth open but doesn't give to the pressure.
As for the gaping mouth - I think it's more likely she is resisting the pressure turning left more so than she is resisting the bit. Work in a halter on getting her to flex laterally and loosen up that left side. She us probably just very stiff. Stand facing her shoulder and gently ask for her to bend in (using the halter first) until she releases to the pressure. Once she has flexed all the way around just start doing that daily, a few times a day. It will really help her. She probably won't be as resistant to turning left if you get her nice and loose. Hope that helps!