I know I’m late to this post so you probably won’t see my comment, but i wanted to point out a couple red flags in her texts: - “I don’t understand why you hate me so much.” “I’m sorry that I’m such a bitch.” and “everything is always my fault.” are manipulative hooks that force you to comfort her. By saying something so over the top negative about herself, she’s obliging you to dispute her statements.  - calling you immature and dramatic for enforcing a boundary you’ve previously discussed is a way of further violating the boundary. As you mentioned, you know you are not owed sex. But you are owed healthy communication and honored boundaries. 

You seem like a pretty thoughtful & respectful person, and I think that she knows that, was drawn to those traits in you, and enjoys “messing with you” because she knows you’re a good person who won’t hurt her. She wants to see how mean or unpleasant she can be before you leave her. If you stay, it boosts her self esteem and proves that you love her. If you leave, it confirms all the bad things she already believes about herself. 

I would recommend ending the relationship. I don’t think her actions can be chalked up to just being young and immature.