What you are reporting isn’t unusual. Dating someone with children from a prior relationship is almost always very difficult, especially if they are teens. The daughter is not going to go away. She undoubtedly senses that you don’t want her around. This probably escalates her insecurities and sets off the behaviors you find so difficult.
If you don’t find a way to be comfortable with her, your relationship with her dad won’t survive. Why? Because your boyfriend can’t be put in the position of feeling like he has to choose between his child and you. The loyalty to the child usually wins. In fact, it should.
The girl is just entering her teens. This is a life stage when life becomes complicated. Remember being a teen? It’s normal for a teen to both want to be close to her parents and to want to distance from them. For young teens who are dealing with emerging sexuality, it’s not at all unusual for them to cling to the parent of the other sex for awhile. It’s a way to “try on” being in a relationship that is (hopefully) sexually safe. If that’s part of what is happening, the girl may see you as “competition” for closeness with her dad.
In addition, If she had hopes that her dad and mom would get together again (and most kids wish for that, even when the parents were always fighting or worse), your presence signals that it isn’t going to happen. Furthermore, many kids get clingy when they are worried that their parent will “divorce” them too.
To answer your question: What you do is learn more about how to manage the challenges of blending a family, especially when one of the people is a teenage girl. The girl is not the problem. Your information gap is. My guess is that lacking information causes your anxiety. The anxiety makes it difficult for you to be compassionate and probably makes you want her to go away.
I strongly suggest that you and your boyfriend go for a few sessions with a licensed family therapist (LMFT) to learn more about your situation and to learn ways to handle the girl with sensitivity and understanding. You do need to go together. Blending a family is a problem for both adults — even if one doesn’t feel he/she has a problem. Hopefully, you and her dad can learn ways to be supportive of all the children (yours as well as his) so that together you can build a solid blended family.
I wish you well.