1)
Why did he say "va'Ani Amarti v'Shalvi..."?
1.
Rashi: When I had serenity, I thought that I will never falter. However, it is not in my hands; it depends on Hashem.
2.
Radak: When I had serenity, and the Yetzer ha'Ra did not rule over me, I thought that I will never falter and sin. I did not think that the Yetzer ha'Ra could tilt me from the good way, for my intellect overpowered my nature. I prided myself about this, until I realized that Your help stood for me; refer to 30:8:2:1).
