I also dated another severely selfish woman. In that relationship it was much like the sibling poster mentions. There were high expectations of me, but even minor expectations of her resulted in her playing the victim or making lame nonsensical excuses. There was no reciprocity, zero respect, and no concern for the relationship as its own entity.
What both these women had in common was a desperation for approval and attention from others. This need was so large that I could not possibly fill it and I was ultimately made to feel that I was only a giver of attention and a tool to aid in the acquisition of more attention from strangers.
This type of thing is very obvious to me now, but when I met my ex-wife I was much younger and pretty clueless about women in general.
You can't compete against someone else's selfishness. You will lose every time. The only thing to do is to walk away, but in the context of a marriage, it's not easy to do.
My ex is still indescribably selfish five years later. She still wants to appear to be a good mother at the expense of actually being a good mother. She still tries to sacrifice my career and passions so that she can suit her whims. She tries to push our daughter on me during her custody period all the time (after fighting for more custody and costing us both $$$). And she still blames me, through a series of seriously twisted thoughts, for the dissolution of our marriage.
During the marriage, surviving her selfishness was the only thing possible. It wasn't that I wasn't focusing on my health and happiness. It was that those things were being taken from me more quickly than I could generate them. And I had to protect my child's psyche from her as well.
I don't think anyone would describe me as a "nice guy". I have a very low tolerance for bullshit and have no problems saying exactly how I feel or saying no. But I expect a partner in a relationship, not someone who I have to grab by the scruff of the neck in order to get them to not be a shitty person.