What is the real problem behind the bitterness?

My eve sisters, I need your insight. As a stepmom to a really awesome child for a few years now, I'm still at my wits end dealing with the child's mother. She continues to bad mouth me. She makes fun of the fact that I may be unable to bear children, and when her child comes to visit, she puts so many questions into the child's mind that it annoys me. I've tried to reassure her that I'm not EVER going to be the child's mother but that I love my stepchild and I want us to bond. I've told her that it's time to grow up and that she should be happy that her child's father married a woman who treats the child well and that in the end, the most important person in our blended family is her child. She and my husband are co-parenting but she doesn't really do her part but we never let the child hear our discussion about her. She after all these years says that she should have been the one he married because she did something that no other female did 4 him. Anyway, what else can I do??? I've tried reaching out to her to tell her that we are FAMILY and I want peace but she's still bitter after nearly 9 years. Any advice??? Thoughts???? Comments????