Maybe a happy mouth bit, they’re rubbery and can be flavoured, or something with a little leverage if you want assistance in her dropping her head
She’s not strong , ideally I would prefer a less servere bit . She tends to also shake her head a lot , Iv had her back and teeth checked and they are all perfect . She does soften but with a lot of effort !
What problems are you having with him? How is he not responding?
Depends on how your horse goes is he too strong or is he just playing on it ?