Unrequited love is the heart of this poem, and the speaker sets it out as a string of small, painful truths. Almost every line ends with the same word, "hurts," so the repetition feels like a count of wounds, or like someone who can't stop returning to the same ache.
The speaker loves someone who doesn't love them back. They hide their sadness behind a smile, wish the other person well even when it costs them, and feel left out of the person's important moments. The line about "more than a decade" suggests a long attachment that fell apart quickly, "in 10 seconds." Another line suggests the speaker feels the person has become like others who left.
The mood is raw, tired, and sad, but also honest. The last line steps outside the pattern and admits that even this long list captures only a fraction of what the speaker feels. That ending makes the pain seem larger than the words can hold.