Selfishness

The cause of all divorces is selfishness. If both are willing, two unselfish people can work through difficulties and problems. Communication and honesty only happen to the extent that both parties to the marriage are willing to be unselfish. Root causes for being selfish are often born out of self-preservation. Unresolved childhood trauma can result in dysfunctional behavior while trying to protect oneself from additional harm. Honest communication opens you to being vulnerable.
Here is a link to a website which I found very insightful after my divorce:
https://www.liveabout.com/an-experts-experience-co-parenting-with-a-narcissist-1103119
It is helpful to think of how you would deal with a child. Books on parenting, sadly, can be a great source of information because communicating with someone that has experienced this kind of trauma is similar because they are often stuck at the emotional age where the trauma occurred. If you are not religious, the bad news is that someone who has been wounded this deeply will never be normal and will need to learn coping strategies to use for the rest of their life. They suffer from a permanent emotional disability which will result in the sabotage of any normal healthy relationship as they try to recreate the past dysfunction so they can act it out. They will often be doomed to feel normal only if their current relationship mirrors this past dysfunction. In this case normal to them does not mean being happy, it means they feel at home. If you are not cooperating in their unhealthy dysfunction, they will likely move on to find someone who will. This is not your fault and you cannot fix them. If they choose not to get help, there is nothing you can do. It is up to them.