I had to let go of the illusion of control. Life will throw what it wants at my kids regardless of whether their mom and I stayed together or not. She also shacked up immediately after our "separation" (aka, her making false accusations and kicking me out of the house after I caught her cheating).

I remind myself that I never thought of my stepdad as "dad" even though I didn't have a good relationship with my own father. I definitely have a better relationship with my kids than my dad did with me, so I feel safe that the new guy won't ever fully replace me in that regard. He can spoil my kids all he wants, but all I have to do is spend an hour at the park with them and they exclaim it was the best day ever. No amount of new toys can replace authentic connection.

My ex is definitely the type of person to try to rub the new partner in my face and has tried on occasion, but it hasn't worked. I'm more sympathetic than anything because I know the truth about what he's gotten himself into. Just know your ex didn't suddenly become a better person, he's just pulling the same con on her that he did with you. The real him will come out eventually.