Jbird, I haven't been on the thread from the beginning so forgive me if I repeat some stuff due to skimming.
You write "Is it possible that I, too, am polyamorous...?"
Yes, it's possible, and you will figure it out if you pay attention to yourself.
1. When you are in a relationship have you ever met another woman and really been fascinated by her -- herself, her personality, her quirks and intellect and emotions, and not just her body? In other words, have you wanted multiple relationships and not just multiple sex partners?
2. Imagine that your girlfriend -- who's been in abusive, crappy relationships in the past -- tells you that she met a gentle, considerate man who makes her feel happy and whole...and she wants to date him but be with you as a full-time partner as well. Do you think your immediate reaction would be jealousy and anger, or would you feel kind of happy that she met someone who helps her feel good about herself?
(I'm not proposing a litmus test for polyness, just tossing out a couple of imagination exercises which may shed light on your feelings about poly.)
My own response to question #2 is a poly response. For question #1 I'm not so sure. So: it's quite possible that you could be monogamous in some ways but poly or poly-friendly in other ways.
And therefore you could, theoretically, live happily with someone who is actively poly while not being poly yourself. Obviously: MonoVCPHG lives that way.
But you gotta know yourself, know your girlfriend, and talk, talk, talk. Poly is not sleeping around, though there are degrees: some poly relationships are more casual; some are so deeply committed they are de facto multiple marriages.
One further question you might ask yourself: why has your GF been with abusive and crappy men? What drew her toward them, and is that still a factor in her personality? If you do have an open relationship, do you think she might be drawn once again into a destructive relationship? What would that do to the relationship she has with you?
Be attentive, be careful; and always remember that we on the forum don't really know either you or your girlfriend...you're the guy on the ground and in the situation. Anything I've written could be completely wrong for you. Good luck!