I'm sorry you hurt. BREATHE.. and then? Could deal with the problems one at a time.
Problem 1: Her Cheating
She also claims when you love someone you should not cause her pain, and since i'm undeniably in pain for some of this process, then the entire thing is wrong.
For the cheating affair -- YES. Tell her it DOES cause you pain as you struggle with the ethics and being disappointed in her and wish she would end it. It isn't her loving someone else -- it's the cheating-ness of it. It hurts you that she would choose to hurt people -- you, and the affair partner's spouse. That behavior doesn't become or flatter her. Ask her to stop it because YES... it hurts to love her knowing she can, will and does unethical things.
Ask her if SHE is after reassure that you value and love her when she asks you things like
"urging me to acknowledge the fact that it does hurts me when she's having sex with someone else- so how can I be as "OK with it" as I claim to be? "
For all you know she's struggling there and "trying to make you jealous" and when you are not somehow that "proves" that you don't love her. (And even that is weird ways of treating people. Note it's a possible reason, NOT an excuse.)
Problem 2: Your Fear
Have you told her your worries? That you are fearful? And that you worry that the next time she falls in love and feels the need to choose, you might lose to NRE?
Could talk about how to handle NRE in future so you get the reassure you need. How to cope with "trust and betrayal" when she's engaging in behavior that destroys trust in the cheating partner's life and other relationship. How are you
supposed to feel about the safety of your OWN relationship with her since she's a party to shenanigans like that? This time you knew. Next time will she keep it in the dark from YOU to "feed her dark side?"
Can she get her "dark side" fed in other ways that don't compromise ethics?
Spill it out on the table and sort yourselves out. Holding back is NOT getting you what you need. And if it's going to suck either way? Maybe as well spill it then -- and try to arrive at something. Because holding back arrives at nothing.
When you Open a relationship, you pay full price of admission. There's no discounts.
Problem 3: Your relationship with yourself
Could soul search a bit here.
What is the fear of this relationship ending about? Do you know your dealbreakers with her? Could she engage in behavior that would lead to you walking out the door? Are you clear in your mind what those lines are? Have you made this known to her crystal clear?
Because there's two kinds of freedom -- freedom to and freedom from. Like... There's freedom TO have a polyship. There's also the other kind of freedom -- Freedom FROM crazyiness.
Know your own boundaries. Then YOU can make choices for yourself
that serve your best healths. You are not "along for the ride" in polyship.
Could become more clear here with yourself and how you talk to yourself about this when you are thinking in your head. What you have here is NOT a polyship. It is you having a cheating partner helping her lover cheat on their spouse. Could not call it a "polyship" because that is what you hope/wish it were. WHY would you support her having sex with the other person?
They started meeting up and having sex, which I knew all about (supported even) though was not all that happy with, because my metamour (hope I'm using the lingo correctly...) is married and cheating, and I have serious ethical issues with that.
Did you know the other person was married BEFORE or AFTER they became lovers? That sentence to me makes it seem like you knew ahead of time. What happened there?
So... you could sort yourself (your relationship with you) out and then sort yourselves (the relationship with her) out.
Hang in there!