Rewatching this ep now, Peggy's delivery of this line:
"You have to believe me that I'll forget about this. I don't want you treating me badly because I remind you of it."
So full of meaning, such a great scene. Don being kind of a dick to her, mostly unintentionally, and her in such a subtle way reminding him "I've done this before, remember?"...and then that last line, a nice little jab at herself, a reference to the way she treats her own child (though we've established the details of this are unclear). This, combined with her delivery of the "Thank you Don," the look on Draper's face as he realizes that she, like him, has succeeded in getting past it all.
Fucking love this show.