I don't know, I'm struggling in a way with my H cheating this year, totally different and long story, but I know I had nightmares about him being with this other woman, and I know I couldn't stop the thoughts if I had wanted to. I think after a lot of time it got easier for me to accept. I had to realize that whatever happened was just sex, and for him that was probably all it was. Now if you were emotionally tied to, or still are, to this person in your past, that would be a totally different situation.
But this isn't something that happened since you met your current gf, so I'm not sure why she is struggling with it so much. Are you giving her any cues that she may pick up on that you are not totally devoted to her? Do you tell her and show her how much you appreciate her every day?
You need to allow her to feel some insecurity, and reassure her, ask her how specifically she would be able to come to terms with it, like what you could do to validate her feelings.
just my thoughts, I don't know