Pretend your best friend has a drug habit/drinking problem and they have been lying to you and hiding it. Maybe you saw things that gave you the idea but for whatever reason you chose not to see it for what it was.
As a result of their problem, they begin lying to you, maybe stealing from you, or they stop being dependable, blow you off, etc etc. You have several arguments about the lies, the stealing, the irresponsibility. But none of those make a difference because they are only consequences of the main problem.
Dan and I have been going back and forth for years over the results/repercussions of his actions. But never, ever have we truly discussed the root of the problem. But it has been there all along. So for me to walk away from him without addressing it to me, would be like realizing that your friend had a drug problem but you walk away without addressing it because it is "too much water under the bridge".
Now I do not think I am all-powerful. I cannot fix his problem. It may well blow up in my face. He may deny, deny, blame, blame. But I will never know until I say something. And I couldn't live with myself if I didn't say something. This man will always be the father of my children and he is supposed to be the model of a man for my son. If i reach out and he rejects my effort, then so be it. But I will know I tried.