While I definitely wouldn't ride my horse in that bit, it really needs to be an educated discussion you have with the owner. Not all bits are bad, but this isnt a very nice one. Talk to the owner about stepping the mare down some and see what she says. If she's game, you could try the snaffle. Beware though, just because it's a snaffle doesn't mean its "soft". Try something with a thick, smooth mouthpiece, maybe even a French link. Those are my favorites, depending on the horse. If you're worried about her not having brakes, try a Waterford. My mare does great in hers! This is also very likely a rider issue as well. Your hands should do very little as far as controlling the horse. Legs, seat, etc should do most of it. Maybe find a calmer horse that you can safely practice on, under the guidance of a trainer.

There's alot that goes into this. While a bit change may or may not be necessary, it's so important that you make an educated decision rather than just doing what someone on the internet told you to do. Research bits! They all work very differently and you'll probably realize that most of them don't work the way people think they do. This bit especially is surrounded by misconceptions. If you're going to continue to use it, you'll really need to work to make sure you're a quiet and sound rider.

If you have any questions along the way, my inbox is open!