Rashi #1: From then (when I asked to be tested) and onwards, my steps did not veer from Your path.

Radak: Tamoch is Makor (like an infinitive) in place of a command. The same applies to "Shamor Es Yom ha'Shabbos" (Devarim 5:12), "Haloch k'Karasa" (Yirmeyah 2:2). Support my steps in Your path.

Malbim: Ma'agal is a circular path. Sometimes one must turn from the straight path to the circle that goes around, until via it he reaches the straight path. E.g. mercy is the straight path, and also humility and shame. Sometimes one must veer to cruelty, pride and arrogance against Resha'im, those who anger Hashem and mock Ovdei Hashem, or when one needs to make a Hora'as Sha'ah (special temporary ruling) that opposes a Torah law, e.g. Eliyahu on Har ha'Karmel. For this, one needs Hashem's help lest he veer from the path, since he goes off the straight path to the side. David says, also when I needed to go on the circular path, also then Hashem supported my steps, lest I falter. Also then all my deeds were l'Shem Shamayim; I did not fall from the path of happiness and perfection.

Rashi: Refer to 17:5:1:1.

Malbim: Refer to 17:5:1:3.

Radak: In some Seforim, there is a Dagesh [in the Tes]; the root is Matat. In some Seforim, there is no Dagesh; the middle letter of the root (Vov) is silent.