Hi, Will. Welcome to the forums.
BrotherMan, I'm about to tell you some things you already know. Please forgive me if I'm stating the obvious.
You are being manipulated. Your ladyfriend is not dealing honestly with you. (I am assuming she's not mentally ill.) She does not want a polyamorous relationship, or she would not sabotage your efforts in that direction. Look at her ACTIONS, not just her words. People can SAY anything, but our actions reveal our heart.
As long as this manipulative, dramatic behavior is working for her, she has no reason to change it. If you want her to change her behavior, you must change your response. In other words, when the drama stops working, the drama will stop.
She's happy with the status quo and you're not. You must carefully consider the situation, and decide whether your relationship with her, as it currently stands, is meeting enough of your needs, or whether the potential is there that it can be re-worked so that it meets your needs in the future. Once you've decided that, your next move should be clearer.
Best of luck to you, whatever you decide.