She is not poly because she is not honest. She cheated and lied. You decided to work with her to polish the results of her lying and cheating. Your efforts gave her something. It gave her the "balance" that works for her. She wants to be with him a lot more than she wants to be with you, and she is. At this point I would go even further than that and say she wants to be with him and not with you at all. She is obviously getting something from you being there that she wouldn't be getting if you left her (is it money? is it a place to have sex?), so she's not going to tell you she doesn't want you at all.
She has no incentive to give you more of her time or attention, because you've clearly shown her you'll ask for more and accept it when she says no. The three of you are going to continue doing this dance until you grow some self confidence and see what is really going on here. You're being used. My suggestion is to let them have each other and move on with your life. The funny thing is, even if you have the confidence to get on with your life, once you are no longer providing for them, they may not want each other when they have to experience the work of providing for themselves.
The other guy feels special right now, because he is getting a lot of attention from her. That's hard for you to watch. She will eventually lose interest and cheat on him too. When the other guy looks at you, he thinks he is seeing the poor guy she doesn't want anymore. He is actually seeing his own future if he stays with her long enough, because this dance is the way she lives her life. He just doesn't know it yet. In my opinion, blahblah, the best thing you can do for yourself is let them have each other and move on with your life.