Bloody hell OP. That is so hard. Those feelings are so natural. You are having to get used to a very sudden shift in the reality you thought you were living in.
It’s so hard to get your head around him being both the rock you turned to when you needed him and the snake that given so little care of your mental health he’s made a decision to do something that could damage it (which impacts his daughter too).
I bet once Hitler knew his time was up he cried too.
His sorrow is not about you, it’s about his loss. I haven’t read anything that tells me his focus is on you and your wellbeing. He is sorry for himself.
The pain of loss and betrayal will be immense for you for a while. There is no going round it. But it will get easier. If you have no attachment figure in your life then your yearning for that could hook you back into it and you might stay out of fear.
But all of these awful gut churning, heart aching feelings you are having, will pass.
He has made choices he knew were not right. There many opportunities for him to roll back. He was warned she wanted more. So did he. She is young. Remember yourself at 20. She has MH issues. He has flirted with her, lead her on and then once the high has gone tried to dump her. If she hadn’t have told you, he wouldn’t have. Since you found out he’s minimised it, blamed lots of things, told lies and tried to get you to cover it up.
May be he was supportive of you. May be he has been your rock. But he is not a good man. Good men don’t do the above. He has shown zero consideration of your wellbeing or your daughters and has simply chased the high of orgasm and the ego stroke he’s had. He’s utterly pathetic.
Had he confessed himself, told the truth, nit minimised and blamed others and been totally focused on you and what damage he’s done to you, I might see a way through this with you being happy and loved as you deserve, but his behaviour afterwards tells me he is unlikely to give you what you need and deserve.
Im so so sorry. But as someone else without solid attachment figures in my life, you can find what you need within yourself. You can be your own nurturing presence.
Take your time. I’d ask him to move out at least temporarily. He could come and go to look after DC. But don’t let him fill your head space too much because he will, I suspect, try and manipulate and coerce you to forgive him and have him back. You need to make your decision in the absence of his words. You need support from people who are there for you. Can you afford private counselling? If not go to the GP.
I think you might find that if you research narcissistic personality, he might fit the bill. If he does then learning more about it will help you to see his patterns.
Remember you have an army of wise women here, some who have been through the same, standing shoulder to shoulder with you. You deserve much better than this.