1)
Why did he say "Kamah Yemei Avdecha"?
1.
Radak: He feared lest he die before he sees judgment [against those who pursue him].
2.
Malbim: It is proper that You do judgment against them in my lifetime. How long are my days? My life is short!
2)
What is the meaning of "Masai Ta'aseh"?
1.
Rashi: I should see it in my days.