13-year-olds are reluctant to confide in their parents. One of the benefits of being a "bonus parent" is that you can act as a sounding board for your stepdaughter to talk freely about the things she's struggling to sort out.
I am a "Real Mom." You may disagree. However, every morning, when I wake, get my stepchildren up and ready for school, make breakfasts, pack lunches, and eventually find time to look in the mirror, I see a mom. Actually, I see my mom.
There are few guidelines for stepmothers and stepfathers about how to understand child development or to parent effectively, instantly. They jump into middle of a kid's development and try to make sense of it. Usually with little support.
When you fell in love with your husband, I am sure you had great hopes of creating a happy family that included both of your children, but as you are discovering, blending a family is often easier said than done.