Thank you for your reply. It feels so good to have someone else to share this all with. I've been very much inside my own head lately.
So, you may have hit on something when you mentioned how things changed when I found out there was another male involved. I can quite easily see that this may have caused some if not most of the jealousy.
I don't feel like i've been cheated on though. I agree she withheld the extent of the relationship she was in but I was aware that there was at least another person involved.
I find it very difficult to talk to her about this because I feel like any decision I make affects the other couple too and I'm uncomfortable with that responsibility. I have met them both in the past and they are wonderful people.
I guess what I need to figure out is how to make this work. As you said, it's unlikely that she would be able to revert back to a "just good friends" scenario with her OSO's and I wouldn't even consider ending things with her.
Thank you so much again, this has been very cathartic