@SadSausage44
I’m so sorry that you are going through this it’s so terrible 
There are some tough times ahead for you, but also him which he absolutely and thoroughly deserves. I am going to tell you some things that I hope you can find comfort in in the coming weeks.
At some point your going to be sat alone and you’ll be sad, lonely and broken. Your going to be unable to help but imagine the picture of happiness of his life with OW and her children. Please let me assure you that the cosy family life you will be imagining him having is NOT what will be happening. Up till now their relationship has be all stolen moments and secrets and the lure of that in itself can be exciting. Now their relationship is out in the open exposing them both for exactly what that are. It won’t take long for reality to strike and before long whatever it was that he once found appealing about her will soon fade in to the background like a distant memory, blighted by a haze of her kids, distrust, regret and about a million complications.
Blending a family in such an instant fashion can only ever end in tears. She’s a bloody idiot for introducing a married man she met in the pub who was still with his wife to her kids!! I mean honestly who does that!!??Shocking!! It only going to be a matter of time before the shit hits the fan for him in that regard.
He will be a stranger in her home, the odd one out, the kids won’t respect him, he will have no grounds or footing to do anything but make himself try to fit. Because no relationship or bond has been built with the kids first it’s going to be like banging a square peg into a round hole, it just won’t go. He will try hard as well to make it fit, he has to, and why? Because he gave up you and your lovely life together to do it so he is going to be trying desperately to make it worth it. By the time he has given up and admitted defeat, you my lovely will have moved on and gotten better and living your life and just like you are now he will soon be wishing he never set foot in that pub and ever laid his eyes on her.
So just remember how ever much you might think or imagine he and her a riding off into the sunset together with her kids, that is not what will be happening. In fact I can pretty much guarantee it, and it would not surprise me at all if at some point I’m the future you find him begging to come back home. Don’t let the bastard back!!! He doesn’t bloody deserve you, and you deserve so much better than him.
Keep strong, you will get through this 