The speaker is looking after someone whose mind is slipping, and the odd thing is that she doesn't seem bothered by it. The first two lines turn on that contrast: her ease is exactly what unsettles him. If she were frightened or grieving, her distress might at least make sense of the situation. Her calm leaves him alone with the worry.
The last two lines show what he does with it. He starts "looking over accounts," a small, practical task that takes the place of anything he could say or feel directly. The poem doesn't spell out whether he means money, records, or something more like a reckoning of what has been lost and what remains, and that ambiguity works well. Either way, he retreats into numbers because they can be checked and managed, unlike her condition. The mood is quiet, dry and a little uneasy. Very little is said, and the restraint suggests how heavy the feeling underneath is.