I don't really have a problem with Betty's attitude. I'm not sure where their relationship is really at but I wouldn't be surprised if we found out that Betty is willfully imagining a tranquil relationship for her own sake. Because it's really easier. The events of the second season likely haven't been forgotten. I think the idea of expecting, and raising, a child with a loving husband is very seductive. It's far more comfortable than facing the burden of being a wholly independent and questioning woman. I mean, she tried that out in the second season with mixed results (it was both liberating and dreary) and I think it brought her to a really lonely and uncertain place. Loving and trusting Don, even if it requires a touch of willful disavowal and repression, is probably the easier route for a woman who doesn't quite know how to be alone. Even if she still has her suspicions, she needs Don now more than ever.
And maybe there isn't any disavowal involved and she really and truly believes Don is going to make an effort. After all, that letter he sent her in the second season finale was quite moving, and I wouldn't underestimate its affect on her. Thus, perhaps she is able to both enjoy and believe in a happy relationship with Don. And if so, that's lucky for her, because the alternative would be so bleak and dismal, wouldn't it? Especially with a child on the way. I mean, it's certainly easier than being an indignant and lonely housewife separated from the comforting support of her husband. The idea of Don and Betty Draper is alluring, its Betty's crutch, and at this time in her life, she has to believe in it. Or she at least desperately wants to.
In a way, the premiere exemplifies one of Weiner's driving ideas: people don't change. Don regressed back into infidelity, and Betty has resumed a trusting position wherein she is still dependent on Don for her own happiness and well-being.
There is also the possibility that her happiness is predicated on her belief that she finally made a stand, she scared Don, and got him about as close to a confession as he's going to get. That's a victory in a way, and perhaps it has given her a sense of security and sure footing that the audience knows is actually quite tenuous.