Women have a mating strategy completely different than men, it's called hypergamy. This happened to me, too, after 11 years. I recommend you all talk to these men. Good men, STRONG ties between men, are the only way society can fix his worsening problem. It is true that all women are like this and they all want to "fuck the alpha in a group", but if you stop being the alpha and the other guy passes you up in some particular way that her age tells her to care about, she will go for it. They are ALL like this if they are ABLE TO BE.

My only saving grace? Didn't get married, didn't have kids. Lol, called her my "best friend", I absolutely recoil in horror and cringe to think of it now. Fucking modern women, I swear, useless. We've enabled this. Not anymore, not with me.

To op. Please let me know if you need someone to reach out to. No matter what, drop all ties with her as FAST AS POSSIBLE. It's healthiest for you and will show her that you AT LEAST respect yourself. Do not be mean, do not be nice, simply let yourself feel when you're alone, and when you're forced to be around her for some reason, have total indifference toward her and get business done.