You will get much good advice here from people who have been through the same or similar.
I'm afraid once someone has been involved in an affair the lies start and you can no longer believe anything they say. That's a huge shock - it was for me. Unfortunately it's almost certain that he is not telling the truth. I don't think anyone leaves a family for someone they have not had a fairly involved (physical and mental) relationship with.
IMO, I don't think it's worth pursuing the truth. He has nothing to gain by honesty and neither will the truth bring you any relief. I still do not know all of the truth behind my ex's affair and our break-up to this day. Personally, I would advise that you act as if he has been having a long sexual affair and had this carefully planned, rather than driving yourself crazy trying to get him to admit the real course of events. You will waste a lot of time and energy.
But from now on you need to look after yourself. He is not your friend. If you can afford it, I would recommend getting a counsellor with the purpose of helping you through this.
For what it's worth someone who leaves their partner and children for the thrill of an affair (and this is her too) is a sad and deluded person indeed. They are seeking something that is illusory and the relationship will likely fail anyway. However, that does not mean he will come back and everything will be okay. I think one of the saddest things is that once this type of wound has been inflicted, it can never be the same - it's over.
As a friend said to me during my breakup - "courage, my friend - courage". You will get through this. Day at a time.