Chapters 42-43 look like the same song, don't they? Complete with a repeating phrase in 42:5, 42:11, and 43:5. Sometimes I forget Scriptures were not written with the chapters and verses included. That is something added in much later. Does it matter? Not really. But there are times when the phrasing suggests no such break was necessary.
I love the verse though.
Why are you cast down, O my soul,
and why are you in turmoil within me?
Hope in God; for I shall again praise him,
my salvation and my God.
The psalmist acknowledges that they do not feel like rejoicing...but that is not good enough. We serve an Almighty God, Creator of the universe, Timeless and Immutable from everlasting to everlasting. As such, the psalmist follows up with the conviction that they "shall again" praise God.
What do we do when we don't want to worship? Do we force ourselves to worship or open a new tab and check Facebook? Everlasting to everlasting means God was here before Facebook (gasp) and will still Exist long after the universe has ended. I find that focusing on God's Infinite Attributes brings my small perspective back to where it should be, even if I don't feel like it. Give it a try and let me know if it helps you.