Don doesn't love himself or believe anyone else could love him. That's understandable since he grew up being told very clearly that was unwanted. He keeps trying to fill this massive void in his life with professional success, drinking, and philandering but it all just leaves him feeling empty.
Don needs some serious therapy, which would have been incredibly difficult to come by in the 1960's.
Don't you think the external conditions of his life need to be overhauled as well? Wasn't he much happier when he was carefree hanging with those rich hippies and just shooting the shit?
I feel like the obsession with building and maintaining the perfect nuclear family is sort of a big part of his problem. He's trying to shoehorn and be shoehorned into a life that really isnt his where he's unable to truly be authentic or slow down long enough to get some of that introspection. Ditto for his job. You become like the people you spend the most time with. His work seems like a huge chronic trigger also.