"You're just who you are.."

She is showing her Objective Viewpoint. She is saying and showing she is looking through a historic looking glass at Her former husband. She is taking a big step away from him, detaching her feelings another amount as she mentally divorces him.

She formerly accepted his liberal lifestyle. Now she sees how that results in pain. She is learning something outloud. But she is being more classy than to simply call him names. She is forcing him to dig into his own morality to find out what she is speaking about. He has to fill in the missing concepts. If he can do that, he will feel pain too. If he cannot, he is an airhead or an infant.

I think Howard felt pain.

She could have said, "You are who you are".
Or, "You are You."

Perhaps her phrase does not translate cleanly.
She is using a Pop Psychology phrase of that decade.

She is saying that her bubble image of him has burst. He was more, now he is "just" this. "Just" means a position on a scale. Example: Just this much sugar is available for export.

She is also being cool, contained, and accepting of the problem. She doesnt try to tear him apart.