I think @Have Hope has a point. She is too young to negotiate a relationship problem with you. And she is at an age when lots is beginning to change---the way the world is understood, the growing ability to think in abstract terms, hormonal changes, etc.
Also think you may be over-thinking the relationship with her---
Do talk to your bf about how you feel but I'm feeling, rightly or wrongly, that you want something from her that she cannot give, and that your giving while nice is not without an unspoken expectation of thanks/liking you. (can't remember if you said but does she have a relationship with her mother?) I'm thinking you need to own the feelings you have and respect hers.