One in a Million (1986)

This film is about a teenager who struggles over his purpose and meaning in life. One day, he saves a child’s life and tells his girlfriend that that act may have been the reason he was born. That weekend, while cleaning his mom’s attic, he discovers in her diary that he was never born. At that moment, he ceases to exist and because he never existed, the child he had saved is killed because he wasn’t there to save him.