Fixing her and I

Okay. I've tried getting advice from my friends (like I normally do), but they seem to be stuck on "Dump her". I'm hoping I can get better advice here.

Stick with me, there is a lot of background, and I really want advice from people who know the whole situation

My girlfriend and I have been living together for a little over a year now. When we got together, both of us were polyamorous. I didn't really practice, since my last two girl friends were very much monogomous. She had a couple other boyfriends, and a couple that she had regular dates with. That was all fine by me. Sometimes you've got a lot of love going, sometimes you are lucky to have the one you've got.

I think it was around July that I started talking to a woman that I met online. There was a mutual attraction there, but my girlfriend gave the red light so it was never pursued. I always got the impression that she didn't believe that I was being faithful. She would make sure to be availiable whenever I made plans with this other woman (Yes, her and I are still friends but have never been intimate). She would hover over me when I was sending e-mails or IMing. I let it go, but I have serious problems with a lack of trust.

Things continued. She dropped one of her boyfriends, but was still seeing the other one and the couple. That tappered off after a while, and she started complaining that she didn't feel comfortable playing with others if I wasn't. I told her that I would start looking, but that I really felt more comfortable when it just happened.

Well, eventually it did happen. I went to a New Years party with some old friends of mine, and after a lot of drinks ended up sleeping with a friend I've had for years. It was a little ackward (Until then we had always refered to each other as brother and sister), but we realized it was something we wanted to do for years and had just been afraid. I told my girlfriend what had happened, and she flipped. She put the red light on that friend of mine, and basically asked that I don't have any other activities until she could come to terms with it.

So, last month she told me that she was fine with me sleeping with other people. I had no real intention of doing so after the last blow up, but I agreed just to keep things simple. The first person I talked to (Who I had no intention of sleeping with) was immediately vetoed, along with another jealous fit.

I told her flat out that I was not going to see any other people. It seemed like the right decision to make. She has been going on ever since about it, along with accusing me of cheating on her.

Now. I don't want to break up with her. Aside from this little thing, she is a really good person. I know that a lot of this is me screwing up. I should have seen the signs that she would not be okay with me sleeping with other women, and never put myself in that situation to begin with. What I need to know now is how to fix it. How do I convince her that I am not cheating on her, and that I care about her more than I care about being poly?

I can fill in more information, but I think I've rambled on too much already.