Unagie, thanks for bringing this up. It is a concept I have been thinking of a lot lately as well. Like you, when AP enters my mind it is more of a "how could I?" icky feeling. Any sort of reminiscing or fondness is long gone. However, I agree that headspace is headspace, no matter the nature of the rumination.
Some things I do that may help you: when he enters my mind, sometimes I involuntarily shiver and shrug my shoulders. When I DON'T do this, I make myself. It feels very symbolic as a way to rid myself of this negativity and nastiness. Another thing I do is immediately replace the thought with one of my BS. And I try as much as possible to put it into positive action, such as sending him a loving text or email or, if we're together, telling him how much I love him.
And I may get 2x4s for saying this, but please be gentle with yourself. Your AP is someone you presumably spend a lot of time with for a period, and is someone you devoted the majority of your headspace. Even with vigilance, it can take awhile for this to fade. Especially when the A is front and center in your life, and you are likely talking about it frequently with your partner, and in IC, MC, etc. It is just an unavoidable topic, and one you need to retrain yourself to have different thoughts about.
I don't know if this helps or not, but it is what I feel is working for me. Patience with the fact that there is no on/off switch in my brain, and also responsibility for the fact that my thoughts are within my control to redirect and change.