What is the meaning of "Ach Bi Yashuv"?
Rashi: I am constantly stricken. All His Makos are on me.
R. Avigdor citing Eichah Rabah 3:1: Even though Hashem is angry at His servants, the Tzadikim, in this world, He returns and has mercy on them in the world to come.
Ri Kara: All nations sin, and the only nation that Hashem punishes is Yisrael 1 .
Surely he means that Hashem constantly punishes only Yisrael. Every nation is punished when its measure is full! (PF)
What is the significance of "Hofech Yado"?
Ibn Ezra: He turns the hand and strikes me. If "Evraso" (1) is Hashem's anger, "Yado" is Hashem's Makah 1 .
Palgei Mayim: There is no reason to despair of Ge'ulah. Hashem's way is to reverse His hand to benefit. If we will repent, He will return to have mercy on us.
Magihah (in Toras Chayim): If it is the enemy's anger, Yado is simply his hand.
Is this specifically during the day?
Rav Sadya Gaon: It is all the time.