1)
Why did he say "Tza'ir Anochi v'Nivzeh"?
1.
Rashi: This is like "v'David Hu ha'Katan" (Shmuel I, 17:14). I am disgraced, to lower myself by those who engage in Torah, in order to learn.
2.
Radak: I am young, and disgraced in their eyes, because I engage in Your Torah.
3.
Malbim: Even though I am young and disgraced, and I have no honor from doing Mitzvos, I did not forget them, for I do them from love, and not for honor.
2)
What do we learn from "Pikudecha Lo Shachachti"?
1.
Rashi: I did not forget to esteem and delve in them.
2.
Radak: I did not forget them like they forgot them.
3.
Malbim: Refer to 119:141:1:3.
