The speaker is looking back on all the nights they gave to thinking about one person, and wondering what else those hours might have held. The poem is built as a single sentence, and it only reaches the word "you" at the very end. Until then, the lines pile up quietly: the nights, the million other things, the possibilities that went unused. The reveal lands like a small confession.
The feeling is a mix of regret and helpless attachment. The speaker seems to see how much time was spent on this person, and perhaps resents it a little. Yet the poem never says the thinking has stopped. It begins with "Thinking about," so the speaker is still doing exactly what they regret, even while counting the cost. That gives the poem its quiet irony.
The mood is wistful and a little rueful. The short lines and plain words make it sound like a private thought, almost a sigh, with no anger or drama, just an honest recognition of how much space someone took up.