The speaker is lying awake with a raw, sudden ache for an ex-lover. They can't understand why tonight of all nights the longing hits so hard, especially when they feel they should be glad about a love they once thought was lost. Instead they sink into grief for something gone for good. What they miss is physical and specific: the taste of a mouth, the way two bodies fit side by side, a hand that matched their own.
Then the poem turns. The speaker decides to write a love song about a girl and a boy who fought hard for each other, and the story they tell is also a quiet confession. Both of them "died" in the end, not through betrayal, but because each changed faster than the other could keep up with. The mood is tender, wistful and a little desperate, like a late-night confession. The plain, broken-up lines make the sadness feel unguarded and honest.