Hi Isabeth the hardest thing about this is that you have to accept that he is voting with his feet, whilst not wanting to lose his family either. You are paying.
Read 'split self affair' by Emily Brown. When a nice, lovely man does this it is because he is too nice and is trying to express the side of him that nobody cares about: his aliveness and his passion. He loves this woman (very hard to read, sorry) but he also values you and his children.
The answer does not lie in the OW or you, but in HIM. My best advice is to act as though he is never coming back. It will empower and strengthen you as a person in ways you will only understand in years to come. (you see, the other side of his 'he was perfect' coin is your excessive neediness and dependence on him. OW is the relief from that do you see.
The thing that we fear more than anything else is abandonment. And yet, the thing we fear the most? Has already happened! And you are STILL ALIVE aren't you?
Take one day at a time, see a solicitor NOW (legal aid for divorce ends in April) and book a mediation appointment. I know you are devastated, but take decisive steps. When he is in a mediation room with you looking at the ending of your marriage, he will feel a lot of pain and it will dissolve the fantasy for him..
But either way, a woman this embedded in your marriage means you do not have a marriage. So let go of that one and let go of him.