The speaker looks back on a quiet fear: that a girl would one day win over the person they care about, leaving them shut out of that person's heart for good. The poem is mostly about holding something in. The speaker keeps asking themselves, "should I tell him?", and the question follows them home every day after being dropped off. The image of fog on a street fits well, because the worry isn't sharp or dramatic. It just settles over everything and makes it hard to see clearly.
The ending shifts the mood. The other person raises the subject first and says, "that's never going to happen." The poem doesn't show us how the speaker reacts, which leaves a small, open silence. We can read relief in it, or maybe a mix of relief and uncertainty about what the reassurance really means. Overall, the tone is tender, shy, and a little anxious, like a private worry finally spoken aloud.