Solost I don't under-estimate how hard this is and I'm acutely mindful that until 3 months ago, your life was very different. I recognise too, that it's going to take more than that to detach from someone you've loved all your adult life. I also understand why you need answers.
However Annie is right that at the moment, your H is so up his own arse with his "soulmate" delusion, that he will be doing everything to persuade himself more than anyone else, that this was the right decision, for the right reasons. The more he hears himself saying the words, the more he'll be inclined to believe it. So getting him to justify this is counter-productive.
Which is why it is going to be healthier for you to believe the version of events that I and others, have laid out for you. Let your own truths be enough for you, because they are likely to be much more reliable than someone who is in the grip of a romantic delusion.
You will get no truths out of him for quite a while yet, I assure you and if there is a grain of comfort in this situation, it is that he hasn't gone down the route of so many adulterers and started re-writing the history of your marriage and blaming you for "leaving him no choice other than to run off with a young girl" 
Whenever I advise you to detach, all I am saying is to "act detached" - you do see that don't you? I know you won't feel detached just yet, because that would be expecting too much, too soon. However the more you act detached, the more you will become detached and you need to give yourself permission to do that, actually.
WRT his statement that he is glad that you are over him, look at the reply. It was that he was glad "as that made it easier for him". Not easier for you and that is hugely significant.
It is only partially true. The selfishness is at the surface of this reply because it lessens the guilt. But the other part, which he won't acknowledge, is that the day you say that and it rings true (it doesn't at the moment you see, because your behaviour towards him contradicts it) will be terrifying, because he will realise then that he really has lost you forever.
So you've simply got to make your behaviour match your words. When you told him you were over him and wouldn't have him back, he knew you were lying. Don't bother saying this stuff to him until you really mean it. Just start acting as though he is no longer any concern of yours.