As many have pointed out, this is often a sign of stress/tension/anxiety and figuring out exactly what it is can be challenging! Some things to think about, does she accept the bit during bridling quietly or does she chomp/yawn/gape? Does she chomp/gape during groundwork? She does the yawning/gaping when you are riding around on a completely loose rein walk/trot/canter, right? Does it get worse when you pick up the reins?
After you rule out physical causes, I would suggest looking for a ‘problem horse’ trainer. You need to find someone who is very experienced with anxiety issues and more often than not, dressage specific instructors are not that. I think you need to take several steps back to just working this bit issue before pressing forward with dressage. This is going to take a lot of time and patience to address… It could take months/years for you and your horse to work through this anxiety.
So many horses have issues with the bit… There is a horse I started working that is so stressed about the bit, even seeing it near his head (not even on his head) results in these weird yawns. I wanted to see what would happen when I put it on so I did, and he yawned and chomped and yawned and chomped non-stop. So I sent him to work on the ground he stopped doing that, but as soon as he stopped moving the yawning/chomping started again. I haven’t tried riding him in a bit… he is telling me very clearly he is stressed about the bit so why add to his current stress level?
Then there is another horse I have been riding for a long time, and after lots and lots and lots of work helping him chill out, I can ride him around on a loose rein in a bit walk/trot/canter/whoa/steer, but if there is two rein pressure, he gets immediately anxious and takes a long time to calm down. It isn’t as bad as it was, but it still happens despite years of work. Since he isn’t angling towards anything require a bit, I finally decided to can the bit and ride him in a halter with permission from his owner. He is so calm in a halter (along with that de-stressing work for years!!!) that my 5 year old can ride him by herself. No bit isn’t an option for you, but just an experience of mine.