you might want to re-read what I said as I had some questions you could ask about jealousy...
I think that it could be a big learning experience for you. You say you are young? This is the time to learn from stuff.. later those lessons become even harder and have a far bigger impact.
I would personally run, but I am not you and don't love her. At my time in life I just wouldn't stay with someone like that. If they haven't figured their shit out by now, then I don't have time and would go to someone who does. I wouldn't leave without giving my full synopsis as to why I am going elsewhere though and you could do that too if you feel so inclined. If you are able to move on at this point that is....
I think your leaving talk could be something along the lines of how you feel that you have been mistreated and why. Try to keep it in terms of "me" and "I" statements. Don't blame her, you were also a participant in how she treats you. You could of stood up for yourself, but didn't (I am guessing it's due to lack of experience). You could tell her you realize that now and because you didn't you don't feel that you could participate in your relationship anymore.
You can say that you are now off to search for someone who will be able to give you more of what you need so that you can give to them too in the way that makes you feel good... Give and take that is even. Tell her what you have needed and then tell her that you wish her well with her other lovers.
If you want to stay and she is willing to work on it all then you could also tell her what you need to feel comfortable and begin negotiating how the two of you are going to be okay with each others loves and how your relationship will be conducted so as to include looking into a relationship with the other woman.
You could invite her to work on this with you, with the understanding that its the dawn of a new era... no more controlling, manipulating, rule setting off her own agenda, and bad mouthing you to her friends who then seem to gang up on you. She has to do this with you and her other partners to a certain extent... it's not okay that boyfriend number one calls you up and tells you just how you cheated... it's none of his business, he was being a meddlesome metamour. This is about you and her...your relationship. Nothing to do with him.
There is also no room in "honest" communication to be forced to show letters that other people give you. Yes, be honest you got them and what the content is, but to show them is a breech of trust with the sender in my books and is disrespectful to the sender. Her demanding them is not okay and is controlling behaviour.
She has a lot to learn about treating others with respect, compassion, caring and patience. Woop-di-do that she has been poly for years, that doesn't make her good at it or a better partner. That is earned through practice.
She needs to realize that the "privilege" is in spending time and being loved by you. It is not her "right" because she willed it. She didn't will it. You gave your love to her as a gift. If she abuses that gift and treats it badly, take it back and give it to someone who will treat you as you should be treated... how you would treat yourself is how you should be treated and how you should treat others.
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