Tehillim 119:35
הַדְרִיכֵנִי בִּנְתִיב מִצְוֺתֶיךָ כִּי-בוֹ חָפָצְתִּי
hadricheni bintiv mitzvoteiha ki vo chafatzti
Guide me in the path of Your commandments; for that is my desire.
Here David Hamelech speaks about the commandments, while earlier he spoke about the Torah (119:34). According to Chazal, Torah study saves one from the yetzer hara (evil inclination), but observance of the mitzvot does not have the power to save one from the yetzer hara.
Accordingly, when speaking about Torah learning, David Hamelech said, "Grant me understanding, that I may keep Your Torah," and so will "observe it with all my heart" without further help. But here he speaks of the mitzvot, and thus asks for guidance and help. "Guide me in the path of Your commandments; for that is my desire." Since that is his desire, his desire will be fulfilled. For one who comes to be purified is helped.
- Me'am Lo'ez