Final answer:
Paul feels overwhelming fear and anxiety on his first day in combat, struggling with panic and a nightmarish sense of unreality.
Step-by-step explanation:
Paul's overwhelming feeling on his first day in combat, as described in "Where Have You Gone, Charming Billy?" is one of fear and anxiety. The text represents this emotion vividly, where he is coping with the harsh reality of war that includes feelings of panic, defeat, and the blurring lines between reality and nightmare-like states. His mental state is characterized by a sense of unreality and a pervasive, inescapable dread that overlays his experience of the battlefield.