How should Hashem guide him in the way of Mitzvos?
Radak: He should open his heart and desire to it, and not err in it, like one who knows the way to go whenever he wants; he does not err in it.
Orchos Tzadikim: Refer to 119:30:1:3. Malbim: Even though he runs in Derech ha'Mitzvos (32), he cannot go in the small paths by himself. There are many details of Mitzvos, and all Mitzvos, many small paths branch out from them. He asks Hashem's help to guide him in these paths and details of Mitzvos and their branches.
Why did he say "Ki Vo Chafatzti"?
Malbim: One who does a Mitzvah due to a desire other than the Mitzvah, e.g. reward, honor, etc., he is not meticulous about the details. However, I desire the Mitzvah itself - "b'Mitzvosav Chafetz Me'od" (112:1), and he does not desire their reward.
