Stay with me whilst I take you on a journey.
Imagine there's a room where you need to meet and agree a good outcome to your current difficulties.
There are four doors you must both go through in order to reach the room in which you'll agree.
However, the agreement can't even occur until you're both in that room, talking to each other constructively.
The other parent is terrible and can't or won't go through any of the four doors, so they may never get to that room.
You are a nicer person and go through, lets say, three doors but you too can't or won't go through the fourth.
Neither of you are in the room.
This is what it means when we say it takes two to tango. You both need to be there or it doesn't work.
You can't fix them (or anyone else) but you can fix you so, unless you have, and you can walk through the four doors and show that you're there, in the room waiting, you can't criticise them. You are as much of the problem as are they.
What do the four doors represent?
They are the four horsemen of the relationship apocalypse. You can read about that here.
Many idioms speak to this phenomena of destroying the 'other' whilst simultaneously destroying yourself:
It takes two to tango
People in glasshouses should not throw stones
To burn ones bridges (i.e to ensure things can't go back to good)
To run in circles (i.e. never get anywhere)
Adding fuel to the fire
Can't see the forrest for the tree's
Go down in flames
That ship has sailed (i.e. deciding the situation cannot be fixed)
Bury your head in the sand
What do idioms' demonstrate? They have arisen out of millennia of human experience and learning. All of the above can apply to the typical post separation parent who cannot move on, continues the behaviours that contributed to the end of the relationship and cannot see that until you change yourself, you cannot ask that of someone else.
Strategic tips;
1) Fix all four horseman yourself
2) Go into the room and wait for the other parent.
3) Show everyone else (ie. courts, expert witness etc..) that you're in that room, waiting.
If you find yourself disagreeing with all of the above, here's another idiom:
"There are none so blind as those that cannot see".
You can be forgiven because you're upset and emotional but, sooner or later, you'll realise that, you need to go back to step one. There's no other way into that room other than through those four doors. And, you'll need to be in that room when the other parent arrives.
They might only find the room themselves because you've shown them it's possible. By modelling it. Think on that.