Also, where the heck did you get the idea that polyamory means having lots of mistresses and lying about it?
So you went from being the wife who was being lied to about the mistresses to being the mistress who is being lied to about the wife? And someone told you that this "is" polyamory?
You could memorize every definition in the glossary and you still would not have the right words to describe this fucked-up situation.
Oh and before people crawl up my ass for judging and assuming, let me tell you a little secret about myself. My mother became involved with a married man when i was 10 or 12. They were together until she died when i was 30 (that's a long time). Supposedly, the "story" was that they slept in separate rooms, had DADT (don't ask don't tell), and were staying together "because of the kids". Well, the "kids" are now in their 40's, as i am. To this day i wonder if that man was telling the truth about any of that. So i think i am very qualified to have a strong opinion about situations similar to this.
That said, my mother was content with their relationship and if he lied, he was a good liar and i'm pretty sure he came and went as he chose, not as his wife decreed. Your situation sounds untenable. The sex must be incredible because i can't think of why else you'd put up with this bullshit.