Why did he say "Ger Anochi va'Aretz"?
Rashi: I am here (live) a short time.
Radak: A person in this world is like a stranger, who has no station in the place. So a person goes from this world, for he has no permanence in it.
Malbim: This addresses [not reaching perfection due to] lack of understanding. I am a stranger in the land. I.e. my intellectual Nefesh is not a resident of this physical world, for it is from another world, and it lives among physical powers that are residents here. They overpower intellect, and it is unable to understand Divine matters, for physicality blocks it.
Why did he say "Al Taster Mimeni Mitzvosecha"?
Rashi: Do not hide from me Your hidden Mitzvos, so I can fulfill them. If not now, when?!
Radak: I do not know the day when I will travel (leave this world, i.e. die).
