Again, my deepest thanks to everyone for their support and insight. Last night I told my girlfriend we needed to have a long talk over IM about our relationship, which consisted of two parts.
The first part consisted of me going through with her the incidents throughout the entire situation and telling her where I was unhappy with how I was treated. This entire process went extremely well, she acknowledged my points in a calm, adult manner and apologized for inviting girl B back into the bed, for giving me a ten minute time limit on handing over the letter, and for slapping me. She insisted that what I had done was still violating the terms of our relationship, that she was not blackmailing me with our relationship to get the letter and that she had every right to see the letter because girl B was both her friend and lover and she felt betrayed. On the whole, I feel this part went pretty well.
The second part involved me putting to her the more general problem of not feeling equal in our relationship, and that I wanted to be able to date other people if it was something she was doing. This led to her angrily telling me that I had no right to be talking to her about this after breaking the terms of our relationship, and then her telling me how poly relationships require communication and control over emotions. She continued to insist that I had cheated on her, betrayed her trust on multiple occasions, and treated her in a disrespectful manner. I mentioned to her that I had contacted other polyamorous individuals and told them the entirety of the situation and what they had thought of it, and she got angry at me for abusing her privacy and insisted that people on the internet are not a reliable source of information. She accused me of not caring about the emotional trauma she'd been through and insisted that the blame lied with me on breaking a promise and violating the terms of our relationship. She told me I needed to listen to experienced players who know how polyamory works, and continued to repeat how I had broken her trust. This part of the conversation got a lot messier and less structured than the first, and at the end of it I wasn't sure what I should be doing.
She said that she spent this morning talking to relationship counselors and is writing me a letter explaining what we both need to do in order to make our relationship work. I know I still love her very much, and the rightness of my cause that I began the conversation with has dwindled quite a bit...I still find myself believing that what she's saying could and may be right, and I'm not so sure anymore what I should be doing. Any more thoughts? I could use some.