So sorry that you find yourself in this position, but each and everyone of us here knows how you are feeling right now.
A few things stand out from your post that I think need to be expanded on
You mention yor 'partner' not Husband. Are you married?
Your username AA35, is that your age? if so that would mean you have been together since you were 16. It may go someway to explain ( but not justify ) why your partner feels the urge to get out there & dip his toe in the water again.
What, if any, discussions have you had since the break-up about why he has come to this decision?
Have you suggested MC to him?
Are you sure that the OW only came on the scene after his decision to leave?
As hard as this is for you now, it has only been a few weeks since all this began. Your partner is still in the 'euphoric' stage of his new life. As time goes on and reality starts to kick-in, his attitude to his new life will change and what you do from now on will have a bearing on this. The best advice is stick with the book, look for what works, stop doing what doesn't & keep posting. Everyone is here to help
FF999
Me 48 W 49 D19, S17, D14 Together 25yr, Married 22yr Me checked-out July 10, back Sept 10 W checked out Nov 10 Separated Dec 10 ILYBNILWY 2nd Apr 11 We're finished + D bomb 17th Apr 11