Someone has been cheated on, and "What I Want" is the speaker trying to make sense of it. The opening lines show a wish to explain everything clearly, to put it into words for anyone who asks. But the poem quickly admits that can't be done. The speaker keeps returning to the same line, "I will never be able to understand," like a mind circling a wound.
The most painful detail is the image of fingers entwined with someone else's, at the very moment the partner claimed to long for the speaker's. The short, blunt lines that follow, "The dirty secrets. The selfish lies. Behind locked doors," read like anger breaking through. Then the questions about whether it was "worth it" fade into "I will never know."
The mood is raw, tired and confused. The final lines are the quiet surprise: despite everything, the speaker secretly hopes the person will come back. That mix of hurt and longing is what gives the poem its honesty.