The tragedy of King Draper is that now that he is "ready" (as he tells "Di"), the tough new girl in his life reveals that she is not. She needs to suffer a bit more for her past misdeeds. And so it goes. The heaping mess of karma, born long before Dick Whitman even entered this world, is still inflicting its vengeance on our late-blooming ad man.
It's not exactly a stretch to say that the episode is structured in three parts to show Don's past with his first wife demonstrating why she remains stuck in an untenable child-like mode, his present with his second wife and why her unstable situation is untenable for him, and his hoped-for future at his past employer Sterling Coo.