So. Ex has got a new home, a new life, a new woman, new plans.. I bet he's even got a new dog. And to his kids, sadly he's still the same Dad as before. No magical upgrade there.
You're imagining a life you didn't have with him, and a retirement you didn't have with him, and then you're "grieving" for an imaginary happiness and content that you didn't have with him.
Worse, you're now having a fantasy that he has a better happier new life with the GF ; his Happy Xmas diary is crowded with invitations from friends etc. (Which you believe because he said it's so. Uh, as if in all those dismal years together he never told any lies or deceived you and made stuff up to gaslight you ?)
So, bottom line. For half your life you let this man rule and control your life. Your thoughts were endlessly centred on him, tiptoeing around him, anxious, wondering what hurt he'll do next to you and your kids . And nothing has changed. He is still living in your head. Occupying all your thoughts, draining all your emotional energy.
Why are you letting him?
Your life now is real, it's all yours and you own it.