What is the meaning of "Nafshi Esa"?
Rashi: I will have intent in my heart.
Radak #1: [I bear my soul] in the way of request - "Nisa Levavenu El Kapayim" (Eichah 3:41).
Radak #2: Some say, [I bear my soul] in the way of a gift - "va'Yisa Mas'os me'Es Panav Aleihem" (Bereishis 43:34).
Radak citing a Drashah: This is like "v'Elav Hu Nosei Es Nafsho" (Devarim 24:15) - just like a worker hopes for his wage, so I hope for Your good and Chesed.
Radak #3: This is like "b'Yadcha Afkid Ruchi Padisa Osi Hashem Kel Emes" (31:6). When one deposits his soul with Hashem [when going to sleep at night], does he not find it in the morning?! "Hashem Kel Emes"! A man deposits new matters with his friend, and he returns old. Hashem is different. A person labors all day; at night his Nefesh is weary. He sleeps, leaves his soul by Hashem, and it returns to his body new and rested - "Chadashim la'Bekarim Rabah Emunasecha" (Eichah 3:23). From this we know that Rabah Emunasecha for Techiyas ha'Mesim!
Malbim: Just like an archer has a target towards which he shoots arrows, so David made Hashem his target for all his requests and desires, both physical and spiritual. Only You are my goal and nothing else, the purpose of what I seek. All my desires are mere means to reach this.