I'm so sorry, happened to me too - 23 years married, I excused 6 months of his crappy behaviour to me putting it down to stress at work, him threatened with redundancy etc. Yes, he was in an emotional attachment with a woman he used to work with; lots of flirty 'banter' is what she considered it, he was utterly smitten and wrote off our marriage, our little family ... She freaked out when I found her phone number on our landline phone bill, called and asked WTF ... Her (33) shrieking "He's 50, I don't want to be with someone who's 50"!
Took him 4 years to meet someone else - he had a miserable time living in a friends spare room and trying online dating (cocklodger, moved in as quick as he could with her). I've not met anyone else - my trust has been so badly battered, I'd rather be on my own and not have anyone let me down like that again - financial abuse, manipulation, the lies he's told about me ....
There's lots of advice out there - firstly, don't beg, you recognize he's done. Someone told me it takes a couple of months for each year you've known him - well, it's 14 years this month and there isn't a day I don't wish bad things on him. The new woman gets a cheat, a liar, an utter sleeze and I feel well rid. Don't know how he turned, can't understand how quickly he turned, but he went almost immediately from my best friend to the absolute worst person I know.
Someone will be along in a minute with the list of do's and don'ts. I wish I'd followed them, lost a bit of respect for myself in trying to get him to understand just how huge his decision was, how it would affect our family ... Fresh fanny, someone put on Mumsnet once, and that made me smile.