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Well done on the advice, Cary.
A few thoughts:
I agree with those who wonder what on earth LW is doing, pondering a serious, permanent relationship with a man who has proven himself unfit to properly care for his own child. This fellow is showing you exactly who he is. Believe him. And move on.
LW, why would you, in one sentence, state that you want no part of being a stepmom...and in the next, ponder taking on the job of playing full-time mother to this young woman? Quit trying to talk yourself into going counter to your natural instincts.
LW, whatever has happened to this girl so far is not your fault nor your problem. You really are not her parent and no-one's asking you to be. She is 16 and already a mess. Believe me, you don't have the natural authority to step in at this late date and make it all better.
Don't do it. This is a situation you can't fix.