Oh, lightbulb.
Love he has been ABYSMALLY CRUEL to you and you are still hanging on for him?
that way only true madness lies.
What IS it going to take for you to get some self esteem and stand up and demand that this twunt either MANS UP and is a proper father/husband to his remaining DS and his bereaved DW or leave you both the hell alone.
What on earth are you teaching your DS about relationships, that you can treat women as commodities, shun them and smarm all over them when it suits and they'll both be lapping it up?
The ONLY chance you have to get him back is to be strong, be independent, be happy with yourself, and live a life you always dreamed of. being clingy, needy and hanging on his every word is tiresome and means he doesn't have to work for it, there is no thrill, there is no chase.
You have to END this relationship with him in order to have any chance at all of getting him back.
But I am willing to bet that once you have done as I suggest, become happy, independent, self confident and loving your life, YOU won't want HIM back.
He certainly doesn't deserve you.
Don't for the love of god send a letter to the OW, 1, she'll know you are rattled, and 2, you seriously think OW have any morals?