The answer to your question can be found in Andy's conversation with his psychologist on January 12th. The therapist asks him if he ever has moments when he feels out of touch with reality, and Andy harkens back to a walk that he and his girlfriend, Keisha, had taken a couple of weeks earlier.
He explains that the pair had stopped at a freeway overpass and had taken a break from their walk, watching the cars go by below them. Andy likens the image of the approaching cars to bullets, which speaks both about the speed of the cars and about Andy's preoccupation with death.
While standing there with Keisha, Andy thinks back to the night of the accident that changed everything. He noticed that the retaining wall was relatively low and would have been easy to climb over. He explains that as the pair stood there, he became "sorta hypnotized" by the sound of the passing cars and became tempted to jump.
He explains that the reason for his temptation to jump was his desire to not feel anything anymore and to have everything "be cool again." He recalls leaning over the retaining wall and feeling absolutely ready to end his life. At this moment, Keisha had stepped in, screaming at him and asking him if he was crazy. She told him to take her home, and when they reached her house, Andy thanked her. The two had not spoken about the incident again.