One who trusts in H'shem knows he has no control over the future or how long he has on this earth. This causes him to be in a constant state of repenting and striving to do the best he can to always be prepared for his final days.
pg 455
לעילוי נשמת אבי יצחק בן משה הלויNefesh Hachaim
The Yetzer Hara renews his efforts every day to make you feel as though your Torah and mitzvot have no value and you are evil. Do not let the Yetzer Hara sway you, know that your service of H'shem is the most precious thing in the heavens, and being careful to follow them as H'shem has dictated in his Torah is imperative.
Prolougue to Gate Four Summation pg 131