The speaker is worried about a lost key, the kind of small, nagging problem that can take over your whole head. The first two lines stay inside that anxiety: the mind is "troubled," going over and over the missing thing. Then the last line changes the scene completely with "the first pear." There is no explanation of how the pear connects to the key, and the poem leaves that gap open.
One way to read it is that the pear is simply what the speaker notices, or finds, in the middle of the worry. It could be the first fruit of the season, ripe and ordinary and unbothered by anyone's lost keys. The poem doesn't say, so that reading is an inference from the imagery.
The mood is quiet and a little wry. The worry is real but small, and the pear suggests a world that carries on beyond it. The poem lets the two things sit side by side and trusts the reader to feel the contrast.