This week's reading from Isaiah asked Israel to look at the rock from which you were hewn (was the rock imagery the reason that this passage was paired with this week's gospel passage?). Back to Isaiah--Look back to be assured about what is going to happen in the future.
Psalm 138 also uses the memory of what the Lord has already done to reassure us of what the Lord will continue to do for us.
This psalm is spoken in the first person singular: I give you thanks....When I called you, you answered me.
But, its message is not confined to just me. Verse 4 says that all of the kings of the earth shall praise the Lord because they have heard what the Lord has to say. And not just kings. The Lord pays attention to the lowly, even more than that given to the haughty.
Then the psalm turns to the future, "When I am in trouble, you will save me from my enemies." Sidelight--knowing God does not mean that we will never suffer. Bad things happen. But God will come to us in those bad times.
Relying on what we know about the Lord enables us to continue to pray for comfort during and rescue from those bad times.