Does anyone know the title

FL and ML are childhood friends. The FL has feelings for the ML, but the ML is in love with his own stepsister. The scene I remember most vividly is a drowning incident. The FL jumps into the water to save the ML's stepsister. The ML dives in as well and makes eye contact with the FL while she is struggling, but he chooses to save the stepsister instead of her. After this traumatic event, the FL becomes cold and begins to distance herself from the ML