harp on one string

To dwell tediously and repeatedly upon a single subject or topic, especially in complaint. Likened to playing the same note over and over again on a harp. Mark is always harping on one string about how little money he makes. I wish he would come up with something new to talk about! I get it, you don't like your job. Quit harping on one string! Come on, I know I made a mistake—there's no need to harp on one string about it!