It's possible to overcome an affair, but only within certain conditions.
If the person who's been unfaithful tells the truth and starts taking full responsibility for his actions.
That doesn't sound to be the case here and unfortunately you're complicit in that right now, blaming his weakness and the OW's predatory actions. It also sounds as though you have been like a bloodhound seeking out the bits to the jigsaw that he could have given you all along, but didn't. Every new truth seems to have come from your efforts, not his. It sounds as though even two months on, you haven't got the whole truth. How do you know they had penetrative sex only once, or that he rejected her straight afterwards? How do you know he finished the relationship? That there were no feelings on his side, either felt or expressed? Unless you have independent evidence, you'll never know because he has lied so much.
All you can really trust in this situation are these: your own memory and the 'things' that can't lie - phone bills, texts, letters/E mails between them and the odd off-guard disclosure he might make when his defences are down.
You can choose to believe a story of a hapless weak man who was feeling low/unloved and was targeted by a promiscuous woman, but I think the truth of it is more likely to be this: your husband was a selfish man who would have taken and enjoyed this opportunity at any time, regardless of his low mood or his feelings about your relationship. Until you and he he admit the opportunistic nature of this and stop putting the blame elsewhere, you will make no progress and it could so easily happen again, given the opportunity.