OPs can take up a lot of head space for people for a long time.
Others don't generally recommend this and it is risky, but meeting with OW totally released the real estate she had in my brain. I was calm, cool, collected, distant, non-chalant. Watching her try to apologize, tell me her sad story and so on - I walked away with the thought of, "Jesus, WH? Really? Her?" She was not worthy of my time/thoughts any longer. I had her built up to be some sort of amazing person (despite having an A) because she attracted my WH, who has generally picked pretty cool people to associate with.
But the OW could also ignore you..or yell at you...or who knows what. All things that could cause her to take up MORE space in your head.
When I have obsessive thoughts (not necessarily A-related), I literally tell myself to shut up, to stop it. I'll even say it out loud. My IC says obsessive thoughts aren't always counteracting by deep breathing or calming thoughts (which I had tried) but, rather, you have to get angry at them. It has been working wonderfully for me. Pep talking myself right out of negative thoughts and verbalizing how worthless they are.
But also understand that obsessive thoughts are a healthy part of the healing process. You have to have that balance of allowing your mind to go there if it wants to - but not allowing them to take over or linger forever.