Uncertain Relationships with Stepchild

My relationship with my stepdaughter has me confused. When she was younger, we were very close. I was her second mother and she loved me regardless of what she was told outside our home that she shouldn't feel or do by others. Now that she is a teenager and has given in to the manipulations (can't blame a kid really for doing that...it was rough for them for a lot of years), our relationship runs hot and cold. Tonight, for instance, she's texting me pictures of the boy she likes in school, texts me a joke, and is just open and friendly. When she was here last, she was confiding things to me that she wasn't telling either one of her parents, most especially her mother. She trusted me to listen and not judge (and help if I can). At other times, she is distant or just entirely ignores communication attempts. Is it step related? Is it teenage angst? Maybe a little of both?

I am looking forward to the day she turns 18 (or thereabouts). I am hoping our relationship will level out once she is an adult, not totally reliant on her mother or worried so much about her that she puts the rest of us at a distance most of the time.