If you are fixated on something, your mind keeps going back to it. You talk about it, turn it over, and cannot easily put it down. People say this when the interest has gone a bit too far — a child fixated on a game, or someone fixated on one old fight. It sounds a little like a doctor’s word, stronger than “interested.” Though actually, in everyday talk it almost always just means “cannot stop thinking about this one thing.” The usual partner is on: fixated on a person, a plan, a mistake.