Simply stated, the Broncos lost this game because Manning was afraid to lose this game. It wasn't the defense. Sure, they let up some late scores, but they gave him great field position on two more early turnovers. You can't ask them to do much more than that. It wasn't his running back, who ran the ball over 35 times for over 200 yards. But the guy who changes the play at the line of scrimmage? The one who was petrified to make a mistake throwing the ball in an unexpectedly close game? You can blame him.
Manning ran his running back into the ground to avoid losing this game. He was afraid to throw the ball until he absolutely had to. That's not normal for him, is it? Against weak competition, he slings the ball and has put up record setting numbers through the first game, first two games, first three games, and so on. But this is what happens to him in the playoffs. It's the same reason he lost to the Colts in his first trip back to Indy. It's the same reason that he is 4-10 against Brady. It was evident when he followed up his four interceptions in New England in the AFC Championship game with an anemic three point performance - the same year he set the NFL touchdown record in the regular season. When the pressure is on, Brady wants to finish the game. Manning wants the game to be over. It's the biggest difference between them.
They're both great quarterbacks, sure, but Manning has built his career by putting up big numbers in small situations. Brady has built his career on what he's done in big situations. This game might not have been won by a Tom Brady touchdown pass in overtime, but there is no question that Tom Brady led them back to win this game. Manning had a chance to step on the Patriots throats after those early turnovers and actually put this game away, but he couldn't do it. It's just not in his nature.