Jill, I'm right there with you.
Now I'm living with a man who has a child from his previous marriage. She doesn't live with us, though.
It is definitely hard. It was less hard when we didn't live together because I only saw her once in a while. Now I see her a lot. She's a perfectly sweet girl, very funny and intelligent. Unfortunately, she hasn't been raised the way my kids will be raised (mostly due to a seriously over-protective mother and an only-child environment where she doesn't even have any cousins). She'd very disrespectful to her father, she gets her way ALL the time and cries when she doesn't (we're talking she cries when she doesn't win a board game - she's 8 1/2!!!

) and sometimes I get so frustrated by her! And I don't take it out on her, I take it our on her father, so that causes some drama there. And she has a wack-job mother, but she's pretty harmless.
So, yes, it's hard. And sometimes I wish I was the only woman in my boyfriend's life

, but I love him and I know that if he never had a daughter, he wouldn't be the man he is today.
The only thing I'm worried about it that I always b*tch "The kids we have WILL NOT act like that!". I hope that doesn't come back to bite me in the butt one day!!!