Okay, I’m rewatching the show and I just realized after the divorce is when Don’s life really becomes a mess. While he did have affairs they weren’t as messy and rarely interfered with his work life. In season 4 he drunkenly has sex with his secretary, begins a somewhat public relationship with Faye (who works with the company), and becomes entangled with Megan. Seasons 1-3 Don would never, in fact he’s appalled when he finds out about Roger and Jane being together. Then in season 5 Don gets bad at his job. He’s constantly leaving and putting all the workload onto Peggy and he has a weird codependency on Megan. Although Don’s life has always been a mess, he was always able to keep it together at work and was considered to be the best. All of this obviously comes to a head in season 6, but this is just something I didn’t initially notice. I do think that part of it is because he truly did love Betty, but mostly it’s because his “perfect 60’s man” mask was slipping. I also believe this is why he married Megan so quickly, although he did treat her MUCH better than he treated Betty (which still wasn’t too great) he didn’t seem to care about her or even really like her all that much. Megan was just a quick fix to make Don seem perfect again. I’m not sure if this was obvious but I would love to hear your thoughts.
EDIT: I want to apologize because maybe my thoughts aren’t perfectly clear (I typed this right after smoking) I am not saying it’s Betty’s fault that Don’s life is a mess at all. What I mean is that after the loss of Betty and Don’s picture perfect family, it seems to become harder for Dick to keep up his Don facade.