What is the meaning of "Yitzeruni"?
Rashi: They guard me.
Why did he say "Lo Sichleh Rachamecha Mimeni Chasdecha va'Amitecha Tamid Yitzeruni"?
Radak: Just like You saved me from this Tzarah, may You save me from every Tzarah. Do not withhold Your mercy from me when I need it. I always need Your Chesed and Emes to guard me.
Malbim: Hashem saves due to mercy, when He sees a crushed Oni in pain, due to Chesed or Emes. Chesed is what Hashem does without merit or a prior promise; Emes is what He does due to merit or a promise. E.g. when Shmuel anointed David to be king, Hashem promised to always save him. Do not withhold Your mercy at a time of Tzarah; Your Chesed and Emes always guard me, even not at a time of Tzarah.
