If you can afford a poor grade, take the class. If you are needing to get an A, then the teacher is trying to help you. You are willing to push yourself beyond their perceived limits and I think that is great.
You cannot excel without the chance of failure. I am a teacher. I teach algebra 2, pre calc, and have taught AP courses.
My biggest desire is a student who is excited to take the subject and willing to work. Good luck.
It's not that they lack confidence in you, it's that your test results probably didn't meet the standard of AP. They're just being real with you and don't want you to take the AP class and fail. But if you're really determined to give AP a go, talk to the teacher again and tell her to give you a chance.
Because it's something you really want to try and experience. Wish you the best :D.