Why does it say "I chose the path of Emunah"?
Rashi: "I put myself to Your Mitzvos" and "I cling to Your Edos" (31) - so [therefore,] I request that You "remove the path of Sheker from me" (29).
Radak: He asked Hashem "remove the path of Sheker from me" (29), i.e. summons in front of me the path of Emunah.
Orchos Tzadikim (Sha'ar ha'Simchah DH ha'Bitachon ha'Revi'i): We must trust in Hashem and pray to Him to help and teach to us the straight paths.
Malbim: Above (20), it says "Garsah Nafshi l'Sa'avah El Mishpatecha" - this is possible only if Emunah is placed in his heart. If not, it will seem that there is no Mishpat in the world when he sees that a Rasha has good and a Tzadik has evil. I chose the path of Emunah, to believe that all His ways are just; there is no injustice. There is another world in which the Tzadik will receive reward corresponding to the evil that he had in the world.
What do we learn from "Mishpatecha Shivisi"?
Rashi: I put myself to Your Mitzvos.
Radak: I always put Your Mishpatim in front of my eyes.
Malbim: I compared [a Tzadik's] reward in the world to come to his afflictions in this world, and oppositely for a Rasha, and I saw that the judgment is Emes.
