I'm soooo glad you're doing better! You listened, and that's great. That is what the Lord wants us to do. I think you're awesome!
My mom, who died just a year ago, was probably the smartest woman I've ever met. She had a practice that drove me crazy,
but it worked. She said, "Pick a line, whatever you want to respond, and say it every time. That way, the person who is [saying something you don't like] will learn that your response will be the same, and they will quit doing it."
My thoughts are that your g/f is probably just stressing focusing on the past. I do that kind of thing too (maybe obsessive complusive at times).
We humans get something in our mind (even if we agree that we won't do something) and we just keep doing it out of bad habits. Sometimes we need a gentle nudge to get out that thought that keeps coming up. Sometimes I find myself saying things that I don't want to say because I'm stressed, and things come out when I'm stressed that I don't want to say and don't intend to say, but I'm reacting to emotions.
So I go back to old habits (that die hard). It can be absolutely unintentional, and I say things I don't mean to say. Sometimes I also know that people can unconsciously want to see if you really mean you're not going to be upset/mad/angry/jealous. So they keep saying things to see if you're honest.
So if I were in your shoes, each time she references the past, I'd say something like, "May be, but we agreed not to talk about our past." I would add the "maybe" part so that it doesn't seem so harsh. You could also say, "Our past is under the blood because we've both confessed it." She will eventually stop.
Pick a phrase that works for you, that is also very kind and loving. Referencing Jesus or the Lord or the Word is kind and loving almost all the time so anything that works that way will be good. Isaiah 43: 18
Forget what happened in the past, and do not dwell on events from long ago.
This also tells us that we should forget, and she needs to be reminded that you've agreed to do that. And if she is referencing them in a negative way, it is even more reason for her to obey the Bible! Negative emotions about the past are very non-productive. And maybe Paul is right, maybe it is a way to open the door to discuss it, but I'd think it would probably just make you upset again.
Sometimes, after showing the person the verse in the Bible, you can just say, "Isaiah 43:18" and she will remember that she is to forget the former things. Nearly all that are following the Lord want to do what the Bible tells them. I'm sure she will too, and it will give you relief. But be prepared! She might say it to you too! 
God bless you. So glad to hear you've been successful in casting out the devilish thoughts in your mind.
