My son left a traditional high school and is attending a high school for gifted teens (part-time) and then taking classes at our local college. He is a sophomore. I know exactly what you are going through. So here are somethings I have learned:
Just because kids are really smart, that doesn't mean they know how to study...if fact, the smart kids get by without studying much at all and then when the work gets hard, they freak out because they don't know why they can't get As anymore. And sometimes their entire identity is based on their GPA. So you need to figure out if she needs to learn some time management/study skills. It can be very hard to get kids like this to ask for help because they see that as a sign of weakness.
Kids who are really bright love to learn and they want to push themselves. Course schedule listings are like candy stores to them. I wish I had told my son that he did not have permission to take AP Chem even though he assessed into it because I knew it would grind him up. He insisted that he could do it. It ground him up. He dropped down to Honors Chem and is doing better, but not until he had suffered a lot of stress. LIke seriously crazy stress.
Kids who are really bright tend toward perfectionism and this is not a good thing. It means that their approach to work load makes things take forever and they are never satisfied with the results. I am guessing that this is where some of your daughter's anger is coming from.
She might be lonely. If she is a year younger than her peers, this is the time where that age difference is most obvious. Also, if she is especially gifted, she may feel frustrated because no one "gets" her and she might not be interested in what they like to do.
BTW, the cracks in my son started to show in Nov. too. I think it was final exam pressure that just made him collapse.
My advice, get a good therapist--one that will work for the whole family. Raising a gifted kid is not for the faint-of-heart, trust me. Two books that might help: Emotional Intensity in Gifted Students and The Gifted Teen's Survival Guide.
BTW, in addition to raising my kid I have worked in gifted education. Message me if you have any other questions.