It's called "living in your head rent-free" which means, according to TheFreeDictionary, "Of a person, to be a source of antipathy or exasperation to one to the extent of becoming a frequent or continual subject of one's agitated thoughts. The term indicates that such a person is able to negatively impact one without any repercussions (with the implication that one should not allow such a person to have such an effect on them)".

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