Hello busybusybee. So sorry you are having to deal with this terrible shock.
You must be feeling totally without a say in this. He's just presented you with it and gone, not giving you a chance to defend your viewpoint on the subject. To me that would be the worst thing about it. The lack of any chance to influence what happens. This is not fair - his first commitment should be to you, you're married with children.
I don't believe these things 'Just happen'. They can't happen without two people making them happen.
You need time to adjust to this new information which will make you feel anger, despair, fear, sadness...your thoughts will twist and turn and it will take you some time to get around to knowing how you want to handle things.
In the meantime I suggest you keep talking on mumsnet as there are lots of people here who will be happy to have you bounce your thoughts off them.
Do you have anyone in real life to talk to as well? You need to feel you're not alone.