I am with Windstar on this one. I don't think that Hardy choose Nancy over you. It's just that the signals reaching him are always "I am not Ok. Help!" from Nancy, while you try your best to signal him "I am OK. Just go and settle things with her." What would you do in this case? You need to change communication here. He can't possibly read your mind and see "Oh, she is doing her best to keep up with this." So tell him and tell her that this is everything, but not OK long time to keep this up. You need to claim your rights in this relationship(s) as soon as possible. Good luck.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.