Jo, passive aggressive people CHOOSE this way of behaviour. They are 'sweet and reasonable' - with a lot of hidden rage. As the Craigslist letter says: if you are unhappy in your marriage, exactly how is fucking some slut going to fix that? Exactly how is that going to make anything better? Have you heard adultery working out well for anyone? If you think you will be the exception, then you are delusional and you need professional help'.
There is nothing you could do (to change this course of events). He decided you were the problem [of course you are!! If it's not you, then he might have to look at himself and his way of negotiating life, which is far too hard. Much harder than losing himself in Miss Youngandslim's whatever]. Do you remember the old game battleships? That is what negotiating with a PA person is like: he has full view of the board and you are making assumptions of where the ships are, because the board is hidden from you.
Starting a second relationship without the knowledge and consent of the person you made promises to, is a very hostile, passive-aggressive act. He has had YEARS to set up his new life, and you have only just found out.
Please be a bulldog over this (financial disclosure). Divorces take absolute ages to get over financially, and considering you have to give him half your pension, which is basically tax-break savings, you MUST strengthen your negotiating arm. It's about the negotiation. Perhaps Saffy can name SHL! So she is 70 miles away, Royal Mail will sort that.
Please, please start caring. This is not 'fighting', it is asserting boundaries to someone who is, and has, been very, very sneaky. This man is NOT your friend. He is going to use YOUR efforts to make his life easy, ie subsidise Miss Youngandslim. Did you go to work for weeks and years for a stranger? Please let him know it's not going to be like that (easy), he is going to have to give a bit in the give and take too.
Read up about passive aggressive people. It will help you shake off your shock and denial.