One of the most difficult realities that you have to face is that the narcissist ex will NEVER change. Their narcissism is part of their personality and it is impossible for them to change. Maybe, with a lot of effort, they could surpress some of their personality traits, but they will never "get better" or want to do the right things. Once you accept this truth, it is easier for you to heal and move forward.
The unfortunate part is that you have children with this person. (probably why you're reading this post) Narcissists love to have children with their supply because it guarantees that you'll be part of their lives forever. Why would they want that? Because they actually thrive in a hostile environment - even one that they create.
Narcissists are like weeds. They're nearly impossible to get rid of and you could spend hours removing them from your garden, but they pop back up to cause havoc and chaos. Even with the simplests of requests, the narcissist will figure out a way to screw things up and cause strive and harm. People who've never had a narcissistic abuser simply can't comprehend how you aren't able to "just get along." They will give you the terrible advice of "just be kind and work it out" or "just communicate with them." But they have no idea of what you're dealing with.
I wish I had better news to share with you on this subject, but I don't. You're stuck with this person being the other parent for the rest of your life (or theirs). They won't change, they won't experience a magical transition to being a better person. But they will make every attempt to blame you and say that you were they reason for them being a total s#@*-head.
I want you to stop for a moment. Right here. I want you to read this next part carefully. Say it to yourself or out loud.
I accept NO blame for my narcissist ex being an uncooperative, loveless, self-centered, uncaring, unloving, abusive prick.
You are NOT to blame for their narcissistic personality disorder. They are a disordered person with an emotionally arrested development. This arrested development probably stopped their emotional maturity somewhere between 8-14 years of age. Basically they act like a child. They throw tantrums when they don't get their way. They are self-centered and selfish.
All this to say, they won't change. Accept that and move to healing.