You're being manipulated by her. She knows that you care about her so she can abuse you emotionally and even physically, by switching between being happy and being angry like flipping a coin.
Look up "BPD", but she's still a minor, so diagnosis cannot be made even if all the signs are there. So don't make it, just a frame of understanding.
You need to stop taking her bait, by being calm yourself and do not react to her antics (if you truely love her and are not feeding your own craving for love from her).
Focus on helping her with the love and material things that she needs and stop at that boundary, not go any further.
Also look up "gray rock method".
Disclaimer: not a professional advice.
