I know what you are going threw. I have been a step mom for too many years, and someday I hope to be paroled hahahaha, just kidding. It is one of the hardest jobs, you will ever have, and I tell you, please don't give up on yourself. My Step daughter, has bounced back and forth from her mom's and her dad's for years.
Nothing is ever good enough.
I am never good enough, my house is never good enough, and my love is not good enough. I am enough for me. She is a child, and no matter what, you are the adult. I hope someday, she will see the pain she has caused, and have a child just like her. Untill then you have got to tell yourself that you have to do the right thing, not for her, not for your husband but for yourself.