How do I co-parent with a difficult ex?
Reduce the number of things that require agreement. Almost everything that makes a difficult co-parenting relationship survivable comes from replacing negotiation with structure, because structure works when goodwill has stopped being available.
You cannot make someone reasonable. You can build an arrangement that does not depend on it.
Make the plan do the work
A vague plan requires cooperation every single week. A specific one lets two people who are not getting along still raise children, because there is nothing left to argue about.
Put everything in the order, in dates and times rather than principles. Exchanges: where, what time, who drives, how much lateness counts, what follows a no-show. Holidays assigned years ahead. Travel notice. Who decides what, and what happens when you disagree.
Every "as mutually agreed" needs a fallback: a request in writing, a response window, and what happens if there is no answer. Otherwise one person can veto anything by not replying, which is the single most common way a cooperative-sounding plan fails.
If your plan is already vague, that is worth fixing, and it is cheaper than litigating each deadlock. What goes in a parenting plan has the full list.
Change how you communicate
Move it all to writing. Text or email, or a co-parenting app that timestamps everything and quietly ends arguments about who said what. This feels cold and is genuinely practical: it removes tone, removes the pressure to answer instantly, and leaves a record.
Keep messages to logistics. Short, factual, about the children. No history, no defending yourself, no responding to the parts designed to provoke.
Do not answer immediately. Almost nothing needs a reply within the hour, and the message you send ten minutes after theirs is the one that gets printed out later. Draft it, leave it, send it in the morning.
Assume a judge may read it. That is not paranoia, it is ordinary reality, and it happens to be identical to the advice for staying calm.
The line you hold regardless
The children do not carry messages, do not report on the other household, and do not hear what you think of their other parent. Not once, not in a tired moment.
This is hard when the other parent is not holding the same line, and it remains the right call. Children who are used as a channel between adults carry it for years, and courts notice which parent did it. If your ex is doing it, document it and take it to the court rather than matching it.
Document, without building a case
Keep a plain factual log: dates, times, what happened. No adjectives. Short entries close to the event.
Two reasons. Patterns are invisible from inside and obvious on paper. And if you ever need a court, a contemporaneous record beats a confident recollection every time. Keeping a record covers writing one that reads as evidence rather than an argument.
Keep it separate from the private processing you also need. Those are different documents.
About the words people search
The vocabulary around this is heavy with diagnosis: narcissist, toxic, personality disorder. The feelings underneath are real and the labels do not help you, for two reasons.
Nobody can diagnose someone they have not assessed, and asserting it to a court tends to damage the person asserting rather than the person described. And the label changes nothing about what you do next. Whether your ex is disordered or simply behaving badly, the answer is the same: specific orders, written communication, documentation, and a court when they are broken.
Describe behavior, not people. "Returned the children four hours late on these three dates" is something a court can act on. "He is a narcissist" is not.
When it is more than difficult
Where a child is being turned against you rather than simply caught in the middle, alienation is a specific claim with a high bar. If the children are being harmed, or you are afraid, this is not a co-parenting problem. What it takes to show a parent is unfit covers what courts actually respond to, and if you are not safe, the safety resources page has people whose job this is.
What a court can order about communication, and whether parenting coordinators are available where you live, vary by state. If the same problem keeps recurring, that is usually a sign the order needs changing rather than that you need to try harder.