Okay, I didn't know you had the vet out, or the tongue/mouth sores. Poor girl!
Have you tried riding her in a halter or mild hackamore? If it is caused by the bit or her remembering the pain of the bit, maybe it would give her time to heal and forget.
Was she shown/trained for WP? Who knows what could have been done to get her to have the right headset. She might very well be afraid of any bit contact at all. They want them to travel on a totally loose rein.
Just out of curiosity, are you riding on a loose rein or with contact? Have you tried riding on a VERY loose rein? Like super droopy reins? Does she get behind the bit then?
I am not a trainer at all. (I only trail ride western.) But I have a feeling it would be very hard for a horse that is not used to bit contact to get comfortable riding with contact. She may be nervous about the bit and is afraid/not knowing what to expect with a new owner and expecting to get pulled on or something. How long have you had her?
When I bought my Fox Trotter she was really nervous of bit contact (the guy told me to ride her in a halter) but I wasn't about to trail ride a high strung horse out on the trail by myself in just a halter so I put her in the gentlest curb bit I could find (she hated the snaffle) and just rode, rode, rode and eventually she settled in with the bit nicely. I think she finally learned my body language and I learned hers and she realized I was gentle with her.
I hope you are able to figure out what is going on with your girl.