ExN engaged to OW

So just two months ago, I (am so ashamed to say) fell for his Hoover. He left OW and said he wanted to be with his family.

2 days. 2 days and he was gone again. Back to OW. Now they are engaged. I mean good luck to them. However it was finding out that he swore on our daughters life that he hadn't been with OW while I was pregnant and we were trying to sort our crap out, that he in fact was sleeping with her.

I don't want him. He actually makes me want to vomit. But this new info provided by him has me feeling worthless all over again.

I can't get away from him as we are about to embark on mediation for custody arrangements for our 10mth old.

I cannot make unemotional decisions wen it comes to him and our daughter. I'm still so angry at him and cannot let go of the intense hate that I have for his involvement in our lives.
It's unfair for me to wish him no involvement in our daughters life as she deserves to know her dad and make her own mind up but far out. I hate that I need to be involved in that.
Owning my mistakes regarding ever being involved with him and getting engaged then deciding to start a family, is a very tough pill to swallow when he (as all narcs do) has moved on while I'm still picking up the pieces and suffering the consequences of my weakness and past with him.

I want to runaway.

How do I accept my reality???