Why does it say "Im Lo"?
Radak: This denotes a Shevu'ah - if it was not so, such and such should happen. The same applies to "Ko Ya'aseh Hashem Li v'Cho Yosif" (Rus 1:17).
What is "k'Gumal Alei Imo"?
Rashi: [I made my Nefesh in front of You] like a nursing baby. Radak - he slowly begins to understand his growth. He goes slowly, and does not rely on himself, rather, on his mother. She prods him to walk and accustoms him slowly. So I put my Nefesh and silenced it, not to rely on its understanding in hidden matters. Rather, it relies on teachers and tradition - the mother.
Malbim: Just like a baby puts all his trust in his mother to provide his needs and conduct him, so I relied only on Hashem, who grants Da'as in investigative matters and conducts me in actions.
What is "k'Gumal Alai Nafshi"?
Radak: I put my Nefesh on myself, not to rely on my understanding. The repetition strengthens the matter.
Malbim: Just like an infant does not rely on himself and knows that he is too weak to do anything, I know that I myself lack strength. I made myself like an infant who does not say a word without his mother.
