My heart really goes out to you.
I really can`t give advice on this, but I can offer some rambling thoughts I have from reading this.
- There is a elephant in the room, so-to-speak, regarding the fact that she left,...and you were then the girlfriend. At this point, you shouldn`t of had to take a 'back seat' to her when she came back.
Why did YOU have to become the friend again ? Why did he do this ?
- The flip-side is, for some reason in your own head, you seem to never of got past the fact you were the 'cheating mistress' at one point. Maybe you felt you didn't deserve to be '#1' with how things started. Maybe there was guilt there, that allowed you to automatically take a back seat the minute she came back.
Does she know you two cheated ?
- Sounds like he took the path of least resistance. He chose her, over you because he knew you would take a backseat easier. Also probably due to his own guilt.
- I understand all to well about being bitter with polyamory. Been there, done that. In your case however, this didn`t start out with 3 people being honest and open. This was an affair. The situation is making you reject the idea, and also lay blame on a easy target.
- My gut feeling is that actions tell a person most of any story. Affairs tend to be things that become more and more complicated as time goes on. Leaving people not only hurt, but also allowing themselves to be treated less then wonderful. If you don`t treat yourself like #1,..the other people in your life won`t either.
I think you should move on, and let this relationship be a learning lesson for yourself in the future. Let yourself grieve and keep learning from the experience, and when you are ready one day, you can start a new relationship in a authentic, and honest manner. With history built in a new relationship, is when you`ll know your real feelings on polyamory.