I don't know if you can fix it, but the next poster followed "The Rule" and quoted (as did a few others), so I wouldn't worry about it.
I don't know if I have much to offer here. To me it sounds like you and your boyfriend have a solid, loving, albeit not traditional, relationship. Can you talk to him and explain your sadness/confusion about your relationship with his daughter, and feel it's interfering with all that you and he share? I don't know you guys, obviously, so don't know how he'd take a heart-to-heart talk about this, but definitely don't become accusatory or defensive in any of this, and try to keep talk away from his DD, focus on your relationship tensions and how to work around them, without blaming the DD. Good luck, and I'm sorry it's gotten to this; my brother had an ugly divorce and I know how a mom can influence her children, and how kids put parents on pedestals.