Thank you for writing. Few people are prepared for what happens when they enter a relationship with a parent of a teen. The problem here is not that you are evil. (You knew that.) Fortunately, the problem isn’t that your boyfriend caves in to a demanding kid. He hasn’t done so. The problem is a very common one (though painful for you).
The daughter is right at the age when, developmentally, girls most want positive attention from their dads. Her dad is the most important man in her life right now. She is figuring out how to be in relationship to men. In healthy father-daughter relationships, the dad helps his daughter feel good about herself as a young woman by listening to her and letting her know she is pretty and smart and interesting. As she develops more self-confidence, the daughter will turn her attention to her peer group. If the dad has done his job well, she will look for healthy men and a healthy relationship.
You didn’t realize it but your very existence interrupted this normal developmental period. You are the “other woman,” with whom she has to share her dad’s attention. She doesn’t realize it but she sees you as competition and is acting that out in a childish way (because she is a child). So I don’t see her as a manipulative spoiled brat. I see her as a kid in pain who doesn’t understand that her dad can give her the attention she craves and needs — and still have a girlfriend.
Punishment for her behavior won’t help the situation. Yes, draw limits. But then her dad needs to have some serious talks with her. She needs to hear that he loves her and that he thinks she’s pretty terrific and that there’s room in his heart for both of you. He needs to make a point to spend regular special alone time with her — even if it is just to go for a cup of coffee or to shop for something she needs or to go for a jog. He — and you — need to “catch her” being appropriate whenever you can, letting her know when you think she has behaved well or said something interesting or funny. The two of you can be a role model for warmth and affection toward each other and toward her.
Above all — don’t take any of this personally. It’s not personal. You could be a goddess and the same stuff would be going on.
I wish you well.