My OTTB hates the bit

My 7 year old OTTB mare hates the bit and becomes extremely agitated when it’s in before I’ve even gotten on. She shakes her head up and down, rubs her head against me, chews on the bit, and sticks her tongue out the sides. She even bolted yesterday the second I took off her bridle and I couldn’t hold her. She’s usually calm, sweet, and listens very well when I’m handling her with a head collar or even without anything on her. She had mouth ulcers maybe a year ago but those were successfully treated, and she has had her mouth and teeth checked very recently as well and there’s nothing wrong there. I’m thinking maybe she hates it due to how she was trained during her racing career? She’s the sweetest girl to do anything with when the bit isn’t in. But her resistance to the bit makes me not want to ride her until I’ve found a solution to this, for both of our safety and comfort. Would a bitless bridle be a good option? Or should I try a flexible mullen bit? Any advice would be welcome.