Having just read your other thread, you are being massively unfair.
It’s clear you resent your ex for no longer financially supporting you and moving on with his life. You admit that that you were happy to stay living together despite the marriage being dead. Your ex has been more than accommodating during your separation, but you have continually dragged your heels, it’s no wonder the divorce took so long.
In your other thread you refer to ex’s partner as the OW when in fact that relationship didn’t start until your marriage was dead and he’d moved out.They’ve been together over 3 years, so this is not a new relationship.
She is making an effort to have a good relationship with your dc, but
I suspect that if she had shown no interest in your dcs, you’d still be complaining that she wasn’t doing enough.
Your dcs are adults, you can’t control them anymore, if they choose to spend Christmas with their father that that is entirely their choice. But I don’t think this is about your dcs. This is about your ex.
You need to let go of all this jealousy and resentment, or you are just going to drive a wedge between you and your dcs, you are going to end up very bitter and lonely.
It’s seems ex and his partner are damned if they do and damned if they don’t.
For your own self preservation you need to let this go.