It sounds like it is easier for you to aim your anger at her rather than at the person that betrayed you. She is not the issue, he is. Take a look at your anger and your pain and get to the root of it. It most likely is not about her.
tired girl hit the nail on the head. I felt anger towards the OW at first because she tried befriending me under false pretenses during the A with my WH. I've known her for 13 years since she was the gf (and mother of my good friend's daughter).
I won't lie and tell you that I wouldn't do a happy dance if I heard she died. But, I no longer let her take residence in my head aside from her being the AP when I think about the A. I focus on helping my R and M. I've come to the realization that it was my WH's fault for betraying me. Yes, the OW was a willing participant, but her role in the A was pretty sad really, my H used her as a cum dumpster. She didn't get any enjoyment out of the A (unless she enjoys being lied to and not receiving any sexual pleasure from my H in return for her blowing him).
So, after some significant time, I can tell you that scratching just spread the itch and delayed the application of a healing balm. In my experience, this "obsession" does fade over time, but it takes some mindfulness and active work on your part. Once you feel you are in a safe and committed place with your WBF, and you have all of your questions answered and are satisfied that he has done his work, I think it's beneficial to be very aware of where you mind wanders and where you invest your energy. For me, my energy was best spent on taking good care of myself and the healthy rebuilding of my M. Too much focus on the MOW just got in the way. (I need to be honest that to reach this "place" took me a good couple of years, if not more, but it was worth it for myself and my H).