Three short lines, and a whole lot of hurt packed into them. The speaker is talking to someone, probably a friend, partner, or family member, and trying to be fair. They start by saying it isn't about something you did. Actions aren't the problem. Then comes the turn: it's something you said. One sentence, spoken once, has lodged itself in their mind and won't budge.
What the speaker feels seems to be a mix of pain and frustration. There's no shouting here, no big accusation. The tone is quiet, almost tired, which makes it sadder. The phrase "stuck in my head" suggests the words replay again and again, like a song you can't shake, except this one isn't pleasant.
The mood is raw and a little uneasy. The poem is a reminder of how heavily a careless remark can weigh on the person who hears it, long after the speaker has forgotten it.