Variant: Don't be afraid of death; be afraid of an unlived life. You don't have to live forever, you just have to live.
Source: Tuck Everlasting: Scholastic Book Guides
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“The fear is simply because you are not living with life, You are living in your mind.”

Source: A dive into the reef's Twilight Zone https://www.ted.com/talks/richard_pyle_a_dive_into_the_reef_s_twilight_zone (February 2004)

“You will thus understand that what you fear is either insignificant or short-lived.”
Epistulae Morales ad Lucilium (Moral Letters to Lucilius), Letter XXIV: On despising death