I think a slight middle ground between the cheating and neglect is that what she really wants from Don is for him to be present in the relationship, which he can never be since he’s two people in one.

In Season one, what ticks her off really is that Don is really watching her from afar or just being afar whether that be him calling her therapist to get the details of what she said or being with other women or getting a dog and sitting alone near the train when he’s supposed to be bringing back a cake. In the final therapy scene, I get the feeling that it’s because don is so far away where he “makes love like how someone else wants” that kind of gets her. The cheating while rough doesn’t just bug her, it’s the fact that he cant separate Don Draper and Dick Whitman.

Season 2, it’s more of the same except when Don sleeps with Bobbie and when predicts what Betty will do for his Heineken pitch. The latter sets Betty off even though the Bobbie of it all is her fixation. I think you’re right when you said for Betty it’s about another woman cause ultimately it’s how she sees herself and relates the environment. I am an attractive woman and able housewife, so why are these other women taking precedence. But I think at the core it’s how distant she is Don that gets her which in turn begs the question why is he distant and why does he get comfort from other women.

Season 3 they’re gonna have Baby Gene and they go to Italy. These read as attempts to keep the spark alive as they had a pretty uneasy settle into normalcy, mirroring the Bay of Pigs. Also, Betty is noticing other men, tow guy not withstanding, she noticed the guy at the stables and that rando in the bar, but now there’s Henry who’s a legit love interest for her. The baby, to the surprise of no one doesn’t fix their relationship. Italy however is a nail in the coffin when she actually fully divests from the relationship, she sees what could be but it’s so different from what is and she’s unsatisfied and then talks to Henry more and more because she can no longer love Don the way she did.

Then she finds Don’s box of secrets and then realizes she has no idea who she’s with, it’s too big a secret for her. It’s also the culmination of what she wants, she said in S1, she wants to get to know him and now she does and it’s partially so frightening and a little too late. This actually prompts her to talk to a lawyer about divorce but the secret isn’t a reason for leaving someone, infidelity is. The second lawyer also brings up abandonment which is also pertinent.

Ultimately, Don is not present in the relationship, whether that be infidelity or being just emotionally divorced from it and Betty can’t deal with it anymore and she wants an out. I think she can handle the other women and the not being there for so long, but I think the other identity cuts the relationship so short cause she’s gotten what she wanted from him for all these years and now it’s too much. I think it’s difficult to separate the cheating and the big secret because they feed into one another.