2 Corinthians 4:16 For which cause we faint not; but though our outward man perish, yet the inward man is renewed day by day.
“Don't lose heart” is another way of saying "faint not”. We may lose heart as our bodies get old like rusty, worn cars. Our better days are spent and we have health issues. We may lose heart when people hurt us or when our load gets too heavy to bear.
While every day brings with it unique problems, every day also comes with its own measure of grace from the hands of the Father in Heaven.
Lamentations 3:22-25 It is of the Lord's mercies that we are not consumed, because his compassions fail not. They are new every morning: great is thy faithfulness. The Lord is my portion, saith my soul; therefore will I hope in him. The Lord is good unto them that wait for him, to the soul that seeketh him.
We “lose heart” when we take no thought of our relationship with God and the “renewing” he offers us “day by day”. Ask the Lord to help you to look beyond this world----- to look to the glory that awaits us in His presence.