Some of the reasons in the psalms for long term optimism
The Psalms are optimistic because they offer hope for all, especially for David. David knows that God will not abandon him if he obeys the law set down for him by his creator and he is right. God repeatedly shows mercy to David in the Psalms and promises to continue doing so if David always worships him. Extending beyond David, God pledges in psalm 25:12-13 that “All the paths of the Lord [are] mercy and truth unto such as keep his covenant and his testimonies. For thy name’s sake, O Lord, pardon mine iniquity; for it [is] great. What man [is] he that feareth the Lord? Him shall he teach in the way [that] he shall choose. His soul shall dwell at ease; and his seed shall inherit the earth. The secret of the Lord [is] with them that fear him; and he will shew them his covenant.” This offers long term optimism in true believers that still lasts even today.
(I feel like I should add what lesson this is, so this is lesson 20)