I'm new to this so I will kick things off with a bit of history.

As you can see in my bio, I'm 40, have biological son (8 years old) & a step daughter (17 years old). I've been married to their father for almost 13 years, so yes, I have been in the SD's life for a considerable amount of time, since she was a young child. One would think that this footnote would help the situation, because I HAVE been a presence in her life......

NOPE. Hasn't had any positive effect. Not even a minuscule amount.