Do the children know about his affairs or could you prove he has had them, in other words would he or has he denied the affairs.
If so you could front it out and say you never had an affair.
Seems fair to me.
He's a hypocrite and a blackmailer to boot.
Is he basicallly using your reputation to hold you hostage to his demands, that of wanting to continue shagging arround and you keeping stun.
He doesn't want you to garner sympathy from the children/grandchildren if you separate so will blacken your character, essentially mounting a smear campaign.
I've got a feeling this sceret will not be quite the grenade you think it will be, your children will still need a babysitter and they will probably think you are both as bad as one another, not great but it's not really the end of the world.
I understand reputation, it is difficult I suppose if you are portrayed as Mother Theresa, but adult children need to grow up at some point.
He really should have stated years ago before you had his children whether he was going to forgive you for this, his current behaviour is worse, you have been a loyal wife since that time, brought his children up and then he seemed to have delivered retriibution on you.
Naw, fuck him, get him divorced and get what's due to you.
Don't be afraid.