I think the conflict is internal. Grant's feelings about Fiona's dementia, together with his jealousy over her new "buddy," Aubrey, cause him to reflect on his feelings about his history of having affairs with students and his love for his wife. In a way, I think Grant is beginning to feel ostracized from his wife, who clearly is immersed in life in her assisted living home and who may not not even know that Grant is her husband.
It's clear that Grant has feelings of remorse and longing for his wife. His inner struggle comes from a both a sorrow and anger at losing the person he loved best and also his guilt over his infidelity and his desire for self-justification. At one point in the story, he excuses his behavior by thinking that he "had never stopped making love to Fiona in spite of disturbing demands elsewhere," a euphemism for the emotional needs of his lovers. If he had betrayed his wife, at least he did not leave her, as others did theirs.
Of course, the irony is that Fiona is the one who left, albeit through dementia, not divorce. She has left a gap in his life that Grant finds chastening and challenging.