Thank you all for your kind words, commiseration and for giving me the kick in the ass I needed to unfollow my ex on the interwebz.
I am pretty convinced that he did start something with this person before we split, which is actually really illuminating when I replay a lot of things he said and did. It certainly doesn't change where we are today, in terms of getting a divorce (though who knows how therapy would have gone if he was all in). It does make me feel less crazy for thinking that there was a big disconnect between his words and actions. Despite my initial incredulity and indignation at this, it has settled into a feeling of calm, and I am grateful for knowing because it alleviates a lot of guilt. It mostly pisses me off because I think he purposefully used that guilt both before and after we split (and kept the relationship from me so that I would continue to feel guilty).
Despite a few pot-shots, I also know this isn't about her, or even the fact that he is in a relationship, regardless of when it started. She could be anyone; she is a stand-in; she is physical comfort and emotional simplicity. I actually feel sorry for her; he probably doesn't even realize he is using her.
So I am continuing to move on, and will tuck this away and call on it if ever he tries to use my guilt again. Thanks, as always, for listening. Being able to work shit out here keeps me from doing and saying things IRL that I would regret.