Discovering a compelling reason

How do we discover a compelling reason that is stronger than “you only live once” or “at least I’m not as overweight as my mother” as means to justify off protocol foods? When I am removed from the temptation and rational, I want to be healthy, I want to be thinner, I want to feel better. When I am faced with the temptation, my primitive brain yells SUGAR! and my mind runs swiftly to “well, life is too short”. How does one silence that terrible primitive brain?