?? flash is at its tightest...?

Depends why she is opening her mouth I guess, but have you tried the Myler snaffle, supposed to be a kind, effective bit by all accounts. Or if you really just want to keep her mouth closed, would a drop noseband help? I have used the KK trainer bit and found it really good for my youngster, so maybe concentrating on her way of going as suggested might improve things x