living rent free
frustrated or victorious
the one-line answer
Occupying someone's thoughts constantly without their consent or any effort.
Living rent free in your head means you cannot stop thinking about someone or something even though they or it does not think about you in return. They are in your head for free. You are paying the mental rent without any return.
mainstream
Your coworker might use this word.
01 / in the wild
hear it move.
complaint
that song has been living rent free in my head for a week.
I cannot stop thinking about that song despite trying.
observation
his ex is still living rent free. she moved on two years ago.
He cannot stop thinking about her even though she has moved on.