A message arrives from someone who once hurt you profoundly, and suddenly, the momentum of your morning changes. You replay an old conversation while brushing your teeth. You carry an argument into lunch. Hours later, the person who upset you may be going about their day, unaware of how much space they are still occupying in your mind.
This is one of the cruelties of resentment: sometimes the person we cannot forgive is no longer doing anything to us, yet we continue carrying them everywhere. The wound may belong to yesterday, a non-existent time, but the disturbance keeps arriving in the present.