have you had a vet look at her mouth?
What kind of bit are you using?
Does she like to jump? I'm guessing that's why she's fine when she's jumping, because she likes it, so if something is hurting or annoying her she has to much adrenaline to notice.
Do YOU have soft hands? If my hands get hard rena gapes her mouth open too.
Does the saddle fit?
If it's none of those things I would probably guess she just wants to do it her own way and gets piss when you interfere, so she knows that she is the boss, and she's just taking advantage of that. She needs to be told(as nicely as you can at first) hey, no, I am picking up the reins and were doing what I want. But only do that after everything else on the list is checked off.
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