It's a horrible helpless feeling when you send them off to an ex knowing they don't want to go. Especially when ex seems to prioritise gf feelings over a child's.
I have similar issues.
Would it help to try and look from a different perspective? Like maybe your ex doesn't want to start sleeping in kids' room, cos it's likely to lead to a regular expectation/reliance?? I dunno, I often have to give up some control cos my ex parenting is vastly different from my own. DC says doesn't want to go but seems to have an ok time.
And the dazzling social life thing. Again I know the feeling but I also remember when I wasn't single but bloody miserable with nob end partner and couldn't see a way out. I try to remember that when I feel lonely. I like the saying "don't compare someone's outsides to your insides"
Anyhoo I'm a fine one to talk. Feeling low too. Going to try for a good sleep. Tomorrow is a brand new day x