The speaker describes a small but important moment: smiling for a reason that has nothing to do with an ex or a former love. Every stanza begins with "For once," which tells us how rare this feels. Until now, even the speaker's happiness seems to have been tied to this other person, either through thinking about them or through "dreaming of a us."
The repetition makes the poem feel like someone checking and double-checking a new discovery. The speaker keeps saying "it wasn't you," almost as if they need to hear it to believe it. The language is simple and plain, which fits the honest, quiet relief of the moment.
The mood is gentle and hopeful. There's a trace of old hurt, since the other person is still mentioned so often, but the ending lands on a clear feeling: "I felt free." The smile becomes a sign that the speaker is starting to move on.