I would think it was when he chose Meghan over Faye. Faye understood him better than just about anybody, including Betty. That's because she understood not only his private life and secret, but also his professional life. She probably would have been about the most stable part of his life if they had stayed together. I would be willing to bet that her stability in character and her help towards his life would have been enough to keep him from having the meltdown which almost cost him his career.

On the other hand, Meghan was a spur of the moment choice by him. They really didn't have too much in common. And once they got married their differences started to show up. Her acting aspirations and the lifestyle it would involve, obviously did not fit with Don. It even seemed like the reason for his spur of the moment propsoal was mostly from the great deal in which she impressed him with her skills among children. (and yet they never had any children of their own). As a career woman, Faye seemed like she would have fit into the role of a weekend stepmother quite well. She would be able to satisfy maternal urges on weekends, but wouldn't need to have her own which would interfere with her career.

I've always wondered if there was one more season would Don and Faye have re-united during his inevitable Madison Ave return.