The war is over, the soldiers have gone home, and Robert Grantham is no longer in love with his wife, in fact he can't stand her. Why? I missed whatever happened between them to cause such intense feelings.
How do you make up he can't stand her anymore? To me it seems more likely that he was anxious because of her falling ill with the Spanish flu and at the same time being more interested in a new maid than was good for him. However, he regrets his actions and feelings bitterly and directly afterwards, so although he is torn between two women for a brief amount of time, I don't think you could say that he doesn't love her anymore or moreover, that he can't stand her. And if you look at later events - yes, there relationship continues to be bumpy here and there, but overall they love and respect each other.
They're basically a normal married couple; they bitch, they fight - really badly in the case of Sybil's death - but they remain happily married and together because, despite it being a money marriage, it worked.