I am not sure when your dday was, but I was consumed with thoughts of the AP for months (she was a friend.) I have to say that maybe just in the last couple of weeks it has started to slack off a bit. (We are almost 9 months out.) I am starting to see her as she is -- a footnote to our long marriage. It helps that we are working hard on our marriage, and it is often when I leave MC that I see her the most clearly. Are you in MC?
Try to be gentle with yourself, and realize that this is a normal stage and WILL pass - I promise.