The poem is built around a simple confession: "I miss you." The speaker counts it out like a tally, "Eight letters, three words, one regret," and the counting is the clever part. Those three words really do add up to eight letters (I, m-i-s-s, y-o-u), so the poem is quietly showing us its own arithmetic before it gives us the sentence itself.
What the speaker feels is longing mixed with regret. The word "regret" suggests that something went wrong or was left unsaid, and that the speaker wishes they could undo it. We never learn who "you" is or what happened, and the poem leaves that blank on purpose, so readers can fill it with their own loss.
The mood is hushed and sad, but also restrained. The short lines feel like someone struggling to say more and managing only the plainest thing. By holding the confession until the very last line, the poem makes those three small words land with real weight.