Why does it say "Ki Chalinu v'Apecha"?
Rashi: Above [the fleeting of our lives, Your anger consumes us].
Radak: Now it discusses Galus. Every man is folly, even those in serenity, and all the more so us, in this long Galus, for Your anger consumes us.
Malbim: After mentioning that Hashem is the permanent abode of the Neshamos - they come from Him and return to Him - the author bewails the brevity of man's life. It is not enough to complete the purpose for which he was sent to this world. Short life is either (a) a punishment for a particular sin, or (b) a general punishment decreed on Adam ha'Rishon and his seed. "Chalinu v'Apecha" refers to the former.
Why does it add "uva'Chamasecha Nivhalnu"?
Radak: The matter is repeated in different words.
Malbim: Af is external anger that punishes. Chemah is inner anger guarded in the heart. In addition to being consumed via the revealed anger, we are confounded due to the hidden anger.
