Looking for a psychological school manhwa (2019-2022) with a tragic "sunlight" female lead and fake-out intro

Platform/Timeframe: Read around 2019–2022, likely on a site like MangaBuddy or Webtoon.

The Vibe: It starts off looking like a standard "last-chance applicant getting into an elite school" story, but completely pivots into a dark, tragic psychological mystery/investigation structure later on.

Key Character & Visual Clues:

• The Female Lead: She has short brown hair, a deeply bright and resilient personality—the classic "girl who feels like sunlight" archetype—but carries a tragic undertone.
• The School: An elite, high-ranked academy where the students wear distinct white uniforms.

Specific Scenes/Lines Remembered:

• The Bridge Rescue: There is a pivotal scene on a bridge in broad daylight where a rescue happens.
• The Line: A heavy, emotionally consistent promise is made: “I’ll become your reason to live.”

Does anyone recognize this? It’s not a generic hunter or system manhwa; it’s a very specific, emotionally heavy psychological drama where the bright girl completely alters the protagonist's trajectory before the story shifts into an investigation/tragedy.