Yeeeeaaaaahhhh I would throw a temper tantrum at my trainer if they ever told me consistent behavior like this was just a temper tantrum. The horse is clearly reacting to something…dental pain, hoof pain, back pain, gut pain or your mixed cues or she lacks confidence to do what you are asking her. As others have asked wether or not she does this on the lunge, with another rider or other tack helps rule in or out the possible causes. Were teeth done under sedation by a vet?? It is just physically impossible to see all the way into the back of the horse’s mouth without sedation and speculum so until that kind of exam has been done I would not rule out something in her mouth, but there could be many many other causes.
@vineyridge I’m at a pretty small barn now, unfortunately. My gut is telling me this is an issue with the bit in some way, whether that be too much play in her current bit or anxiety due to the racing history, etc.
I think I said this before, but I noticed in her racing photos they used a Dexter ring bit. I’m not even above grabbing one of those for $25 just to see how she takes it and then returning it. My current plan is to go to Big D’s (45 minutes away, thankfully) and grab 2 bits, try them out and exchange them for something else if they don’t work. I personally can’t afford to buy a ton of bits and keep them, especially when some of them are running closer to $100. I have a feeling when I find the right bit, it’ll be really obvious. I did just watch a video with Eliza Wallace and she said her OTTBs go best in Happy Mouth/rubber mullens. I’ll be grabbing one of those! Lol.
Just remembered. I also had some good success while foxhunting on a rooting horse with just kicking him on. He was so surprised when I did that that his head immediately came up. Of course that’s not ideal for a show environment, but my horse (also OTTB) only rooted in company.
I would also point out that an ex race horse who raced in tie downs may have tongue or hyoid damage, and that can lead to strange behavior.
I would take the bit out and ride her in a hackamore of some kind. Find some to borrow, probably something with a little leverage. I do this all the time with horses who are fussy. re-school them, then put a bit back in later. Usually works like a charm.
I agree with the hackamore, but not a mechanical one. I would try a bosal (the headstall that western horses use, with just head stall and a lose noseband.) or a “side pull” ( a bitless bridle – the reins come straight from the noseband And keep using it for several weeks. It might take your horse a bit to figure out. If you can use a “not a bit” 
then you try to figure out what part of the bit that is causing a problem.
Since different bits have not told you what you are trying to find out, then it makes sense to take the bit away and find out what happens.
Of course, it may be physical or it might be learned defensive behavior… 
But it doesn’t sound like a hissy fit.
