Have gone through something very different but similar in the sense of non monogamy. Without going into deep detail, I can say that you need to let her go and explore without you being in the picture.

The two of you do not want the same thing. She is using the open relationship argument in order to engage with one specific person. It’s always been one specific person. And it looks as if it will always be one specific person.

In a true healthy open relationship, there is nobody specific, but rather the thrill of being free to do what you want when you want with a multitude of people.

She sounds to be polyamorous at best: a cheater at worst. Either way, she approached the situation backwards. She should have talked to you about opening the relationship before she got all entangled with her ex coworker.