I tried what the above poster did but just didn't love it for my mare. now I have a WTP bit with extended plat I use for daily riding and warm up that I really like, haven't had near as much coughing with it. I need to work up the courage to make a run in it, I'm light handed but still. The last run I made on her in a large arena really took its toll on her when she came out of the arena (difficulty breathing)
I would really consider a good calmer. I would also have a horse tested for Ulcers and EPM. I had a really nice mare in the past that would displace...due to nerves...which were caused from having EPM and Ulcers. My current gelding has displaced with me a few times recently....he has ulcers. Just a thought.....dont try and cover it up with a tongue tie or a certain bit before you find out WHY they are displacing.