It is a year that I have tried in the most gentle way to ask him to come to terms with my decision to leave and help me fix a plan for the kids to soften the blow on them.
Sorry, there's only so much you can do. If he were reasonable and thoughtful, he'd 'help' you, but such things fly over his head or he avoids/sabotages them. The why's don't matter. You are not getting cooperation, you're being obstructed in various ways.
Accept* that he will not cooperate. Accept that he will try to make you feel frustrated, helpless, angry, stuck, etc. Accept that you cannot expect anything except crappy behavior, especially now during the separation phase.
* Acceptance is the last of the 5 stages of Grieving a Relationship Loss.
But he is devastated and cant get out of victim mode, and by refusing to play ball is making me suffer.
He's a big boy - or should be. As an adult he should accept life's ups and downs. However, be prepared for the court not to try to change him either. Court will deal with him as he is, you do the same. Blaming, blame-shifting, playing games and gotcha maneuvers will happen but don't let him or them catch you off guard.
Don't get into shouting matches with him. Venting at him won't have a positive impact on him. Rather, there's a certain amount of risk that he might turn this around on you and claim that you're the one yelling and abusive. How to view this? See him as a master manipulator, don't let him be a puppeteer and pull your strings anymore.
With a person like this you can't 'win' at his game, at least not the way he sees 'winning'. (Think of how a casino operates, although customers - the marks - win often, but overall the House always wins, the odds, the averages, are stacked in the House's favor. Likewise, he sets himself up like that, his game, his underhanded one-sided rules, he makes it so that playing his games will always make you feel you're the loser.) Be bigger than him, don't play his mind games. As others have written,
Don't let your ex live rent-free in your head. Stick to the basics. He does not see you as an authority. And by you trying to reason with him you are letting him grab the controlling hand.
So who is the real authority, the authority that matters? Domestic/Family court. Stop venting at him, it's a lost cause and he might try to use it against you. Yes, there is necessary communication, tell him what you see as the actions to take, then emotionally step back, and resist the urge to reason or argue with him. Look to the court to set up a basic framework going forward. Focus your energy on getting the best outcome in court.
Repeat: You can't reason with him, not while he's using unfair emotional blaming against you. You can't win the Disordered Blame Game. Change your focus, do your best in family court to get the best orders possible for yourself and the children. From the sounds of it, the kids won't do well spending much time with your ex, at least not right now during this initial raw separation period. So don't try to be 'fair' with him and push the kids to spend a lot of time with him. If there are reasons for his time with them to be limited or somewhat flexible (such as the children having the option to return sooner than scheduled) then don't hesitate to seek to have those conditions included in the temporary order and carried over into the final order if it is still an issue.
Repeat: Don't argue with him any more. It doesn't work and will probably never ever work. Negative engagement will hurt you but feed him. Release your frustrations some other way. Take walks, jog, chop wood, clean house, whatever it takes.
And a question: Are you still married? Has anything been filed with domestic/family court? Document these times you pick up your kids sooner. When you do file, you want to convince the judge that there is good reason for any orders to specify that the children's visits with their father are optional or to be cut short depending on his behaviors.
