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This relationship is simply over. Even after the writer leaves him, she'll come back to him at least briefly after "Bill" either dumps her or she realizes he was only desirable from a distance. But that too will be temporary because she and her current boyfriend are simply incompatible. The current boyfriend probably realizes this much more than she knows and maybe it will be a relief for him too if she finally turns the lights off and he can move on as well to someone with less baggage who he is more compatible with.
My only thought or wish for her is that she not try to milk him for part of the house he bought and that she give back the car that his family bought (plus all the other stuff his family and he probably provided her with). She describes him as immature, but from a financial/getting her life together standpoint, she may actually be equally as immature and maybe even more so. He has been mature enough at least to create a little "welfare state" for her where she has a house, a car, and medical/psychiatric care provided where she could not provide those things to herself. She should do the "mature" thing and return those things before she does the right thing and ends this.