Hi devastatedwife,
He's denying it because he feels shameful about the affair, I asked my ex if there was another man, she denied, later she recounted that because she said that she's leaving and we're still cohabitation at this point that she didn't have an affair, a pwBPD will dissociate and alter reality to match their out of place feelings. You can deny it all you want, actions speak for themselves.
I lay awake at night imagining that they are together and holding each other and having this wonderful time while I sleep alone. I cry all the time. I don't know if this part of the BPD or if I really did screw things ups so badly by choosing my son's needs over his.
I'm sorry that you're going through a difficult time He has a serious mental disorder and dissociates that he doesn't have it, he doesn't want to go into counseling himself, granted he does counseling to possibly reconcile, you're not responsible for him going outside of the marriage and seeking another r/s, he made that choice, he screwed up, not you. It's not your fault