Read other letters about this article
because in the end everyone who writes in has a problem for which they need advice. But there is something about this letter that chills me.
Her boyfriend buys a nice house with an ocean view and she considers that she has done her part by bringing in some kitchen gadgets and decorating. Only seeing the daughter infrequently around their weekly dinner suited her fine. She acknowledges she would be the kid's stepmother but refuses to be her teacher or caretaker (pretty much the definition of a parent).
For me, the critical sentence was when she said she would rather do her mother's laundry than Anna's. Clearly she does not regard Anna as part of her family despite having been with her boyfriend for the last four years. Without that essential connection, her becoming Anna's stepmother would be a monsterously selfish act by her and a completely shirking of her bf's responsibilities as a father.
So, yes, LW, leave. You started a relationship when you were twenty five with a man many years your senior. He somehow managed to avoid having his daughter interfer with your romance for many years, but that situation has changed. And sending the daughter away to a boarding school as others have suggested would send a horrible message to the child, as she will know why she was tossed out of the house. But compared to living with two adults who clearly have no interest in helping her make her way through these difficult years... well, I can see why she needs to escape into a fantasy world.