Ah, the script! They ALL say this when caught, every single one of them. I seriously wonder if they learn it in a secret class at school!! Op, it’s utter, utter horseshit. He HAS to say that to you and himself, because the truth is far less palatable. The truth, which he will have to face in time, is that he’s a lying shit bag who has broken his vows, and shit all over his family, for the sake of a shag with a shiny new toy. It is literally nothing to do with you - it’s him, his weak morals and his wandering cock!!!
I’m sure there are things that could have been improved in your marriage, since no marriage is ever perfect, but equally, no marriage has ever been improved by some weak willed dick head sticking it somewhere he shouldn’t. If he wanted to improve the marriage because he was just so unhappy, he could have suggested counselling, date nights, getting a baby sitter and spending more time together - there are a million alternatives to the shit show he has chosen. And yes, it’s not a “mistake” it’s a choice - a fucking selfish one.
Find your anger here op and tell him to leave. If miss shiny new fanny is all that, he can go and live with her in their special ivory tower of luuurrvvvee. After 3 months he doesn’t know her at all and the excitement will disappear once it’s all out in the open and it’s not forbidden anymore. And, let’s be honest, if she is knowingly shagging a married man with kids, she’s a rancid fecker too……At that point, he will be back, crying, realising what he’s lost, blah, blah, which is the next part of the script. It will be up to you to decide what you want to do then. But, for now, repeat to yourself, hourly if necessary, I AM ENOUGH. Hold on, you will get through this xxx