Mother had always told him stories that began, "Once upon a time…" She had gathered her children close, taking turns holding them in her arms to keep it fair, and she had told them stories of people who had braved the world's judgments for love, or struggled against insurmountable odds for the sake of what was right, or saved whole kingdoms with strength of arm or wits or magic.
No matter the obstacles and the pain her heroes faced in their journeys, they always learned something from their struggles, and they always saved the day. Hawke remembered her stories all his life. They kept him going when they lost Father, because adversity was part of life and growth, as was loss. They kept him going when Carver died, because some childish part of his heart insisted that Carver lived on through Bethany.
And Mother's stories were with him when he followed a trail of blood as he might have followed a trail of bread crumbs through the woods, because right up until the moment when what was left of her rose from Quentin's chair, he was certain that he would save her. Just like in the stories.
Hawke had always been the hero in the stories he told himself to get through the life the Dragon Age had set before him. As his mother's spirit departed, he was forced to entertain the idea that perhaps he wasn't a hero after all.
And if he wasn't the hero, that meant that he could lose everything he loved and gain nothing in return.
"Once upon a time" lost all ability to comfort him after that night.