The speaker has just been told that what felt meaningful to them was "only a one time thing." The first lines are plain and wounded: they never wanted to be a fling, and hearing it said out loud broke their heart. From there the poem moves through doubt. Was the other person just entertained? Did they ever care at all? The speaker admits to being scared, which suggests the rejection has made them question the whole experience.
The ending shifts tone. After saying they don't want to forget and can't imagine moving on without regret, the speaker suddenly says "to be real, it was a blast," then ends on a sharp line: "you weren't the first / and you're certainly not the last." It reads like an attempt to take back control, though the bravado feels thin. The mood is a mix of sadness, insecurity and defensive toughness, as if the speaker is trying to hide a bruise by shrugging it off. The simple rhymes and short lines give it a casual, diary-like feel.