“You've got to take the rest of your potion. Harry,” Mrs. Weasley said at last. Her hand nudged the sack of gold on his bedside cabinet as she reached for the bottle and the goblet. “You have a good long sleep. Try and think about something else for a while…think about what you're going to buy with your winnings!”
“I don't want that gold,” said Harry in an expressionless voice. “You have it. Anyone can have it. I shouldn't have won it. It should've been Cedric's.”
The thing against which he had been fighting on and off ever since he had come out of the maze was threatening to overpower him. He could feel a burning, prickling feeling in the inner corners of his eyes. He blinked and stared up at the ceiling.
Harry Potter and the Goblet of Fire
The sentence looks a bit complicated to me. I think the main part is: The thing was threatening to overpower him. But I'm lost here about what "the thing" really is in this context. Any thoughts on that?