5 YR OLD that is So Adament on Misbehaving! Please HELP!?

I have been with my boyfriend for almost a year now, he has a 5 year old daughter, Charity. He works, I have stayed home with her every day since we have been together. Her real mother pops in and out of the picture, but really doesn't impact Charity's life much at all. She has started calling me Mommy now, and she used to be an angel. Now that I have been around for so long she has started misbehaving and having the attitude of a 16 year old. I've tried everything, grounding her, no tv or computer, time outs, put her in her room, . And none of it has worked! My boyfriend and I are out of ideas, we just don't know what to do with her. She is very smart for her age, and she knows what she's doing is wrong, she just doesn't care! But when she goes to other people's houses, they come back and tell us how wonderful and kind she is, and what a big help she was having her over...It is so stressful dealing with her, knowing that she can be good for other people, but chooses to be bad for her own parents. I want her and I to have the relationship we had when I first moved in. But I just don't know what to do with her anymore. Please help!!!