Welcome to SI. Sometimes we want advice, sometimes we get it even when we don't ask for it, and sometimes we just need to vent. If you didn't want any input whatsoever, I reckon you wouldn't have posted this.
Can you fill us in on a few pertinent details? When, and how/why did the LTA end? Does BH know?
What I've come to understand, and hopefully your IC is helping you with this, is that affair sex was like a drug. What AP and I had was, as you said, "chemistry," quite literally. AP was not a special or magical person. He was, I believe, an experienced cheater (and possibly psychopath) who knew which buttons to push, in order to give me that chemical rush and, therefore, get me "high" (and hooked) on him.
I can't stop thinking about the OM when I have sex.
Gently, my cheating sister, you can. You just don't want to. When you're feeling down, does thinking of AP make you happy? If so, that's the drug talking. Detoxing from AP was very difficult for me, and many WS can relate to that, it's a frequent topic on SI. So long as you give AP mind space, you're going to stay addicted.
If you want to stay M, you're probably going to need to kick the "fantasizing about AP" habit. After you get through withdrawal, many of your negative feelings toward your BH may begin to dissipate.
[This message edited by 20WrongsVs1 at 8:33 PM, April 11th, 2014 (Friday)]