LC I don't know if you are still lurking, but I am not surprised you are feeling like this. You are in a new shock phase and don't underestimate the effect of that. In fact, given your profession, it might be worth talking to someone at work about the effect of shock, physiologically and emotionally.
You are having to face up to something you have always preferred not to believe. Your denial was always understandable, because however much an objective stranger could see the truth, you were being told by a RL person, whom you loved, that you had got it all wrong.
You might recall some of my interventions in the post-discovery phase, because I suspected you were being gaslighted in the extreme. What I have found helpful to people who are victims of this, is to get them to focus on incontrovertible evidence. Hence, I suggested you asked for the last 2 years' phone bills. When he refused that request, I was even more convinced that you were being gaslighted and as an objective observer, wanted you to see that your suspicions and doubts were perfectly valid.
As an onlooker, once I suspect (and then get evidence of) gaslighting, my concern moves away from advising about relationship repair and focuses entirely on the mental health of the person whose mind is being messed with.
Hence, my concern for you all these months has been to protect your mental health. It is why I intervened, when ever I felt the gaslighting was getting worse, because what always helps in that situation is another person giving a reality check and reinforcing the truth, as opposed to the lies you so wanted to believe.
So here you are, with a version of events that more accurately "fits" the true story of your life for the past 2 years. At last some things make sense.
But you are in too much shock just yet to do that timeline, to face up to more pain; that he was doing this when you were nursing your Dad. At the moment, that's too awful to contemplate.
But eventually, you will need to do that. You should not make a decision until you have all the bits of the jigsaw and can construct the truth of the last few years.
Having him there while you are doing this is the worst idea imaginable. You need peace and order, to recover from the shock waves and build up strength again to face those demons and the need to timeline more accurately.
In your shoes, I would get some acute help for the shock and then withdraw completely from him, while you are deciding. If you need more jigsaw pieces from him, get them, but make no comment about each revelation.
His physical presence, especially if he is behaving like a victim himself, will compromise your decision-making process enormously, as will the understandable competitive feelings towards the OW. These latter feelings are red herrings actually, that get in the way of a proper decision, based on all the facts.
Peace, order, calmness and for someone who has been gaslighted, a chance to reconstruct the real story of your life in recent years, is what's needed, but only after you have recovered from the acute shock.
Thinking of you (and thanks armbow 
) and offering continued support, if you need it. (watery smile)