Psalm 90 appears to break down into four distinct but closely related sections. The first section (V.1-2) celebrates the eternal nature of God. This God has been Israel's dwelling place in all generations and existed before the mountains. This section of the Psalm even uses a feminine image for God, saying that God gave birth to the mountains. We often think of God as Father and Scripture repeatedly portrays God with masculine imagery but this is one of the rare places in Scripture that shows us that the analogy of a mother to her child can also serve as a fitting metaphor for the relationship between God and God's creation.
The second section (v.3-6) of this Psalm constrasts God's everlasting nature with the transitory nature of humanity. We are reminded that human beings simply return to the dust and that a thousand of our years are like yesterday to God. Humanity is also compared to grass which sprouts anew in the morning but is already fading by evening of the same day.
As if the life described in the second section of the Psalm were not short enough, the third section (v.7-12) of the Psalm compounds the problem. Not only is life short but in addition to this the Psalmist says "we have been consumed by your anger". God knows all of the sins and failings of Israel which leads thier short life to be filled with God's wrath.
What is probably most interesting is the Psalmist response to all of this. One might easily conclude that despair is the only proper response if the everlasting God knows all the sins of our short, almost insignificant lives. But that is not the Psalmist's response. Instead, Moses (this is the only Psalm attributed to him) cries out to God for deliverance. He does not despair that God knows all his sins and the sins of his people. Instead, he asks God how long it will be before God does something about it. He asks for God's loving-kindness and favor and that God's works would once again be evident to God's servants. He asks even that the work of their hands might be confirmed.
This Psalm demonstrates the remarkable confidence that Moses has in God. And why wouldn't Moses have that kind of confidence in God? Moses knew all too painfully of his own short-comings. He knew also of Israel's failings. And yet he had watched God deliver Israel through him with amazing power and unthinkable love. Even though God is infinite and we are finite, even though God knows all of our sins and failings, we can still call upon God to work in us and among us because his love for us and his power to work in us are that great.