But those who wait upon the LORD will renew their strength; they will mount up with wings like eagles; they will run and not grow weary, they will walk and not faint. — Isaiah 40:31

Those who wait upon the LORD will renew their strength—the Hebrew says 'change strength,' meaning God trades your weakness for his power, because when Paul was weak, then he was strong. That strength doesn't come from trying harder but from cleaving to the mighty one of Jacob and refusing to let go until he blesses you. Cast your burden on the Lord, and the God who equipped David against goliath will equip you for whatever stands before you.