Whatever is going on, it feels like inability to handle the class is more excuse than issue. Random possibilities might include...lack of confidence. perfectionism. underlying hidden learning issue. social issues and self-image, in general and/ or around the kinds of kids in higher level math. It's hard to tell if this is normal teen angst, or at least typical gifted issues, or something a little more hidden. But from what you'e said, she likes thinking about math. She's very good at it. And she's avoiding it. What is she actually avoiding?
Another possibility is a poor teacher fit. One of my DD's friends (who is probably DYS material but has not been tested) is currently failing algebra because of a teacher who does not give partial credit for careless errors in the middle of a problem. You are right or you are wrong, and if you are wrong, it doesn't matter whether you don't understand the algorithm or you had a brain fart and said that 5x7=45 during the last step.
Wow. I would fail too.