1)
Why does it say "Same'ach Nefesh Avdecha"?
1.
Radak: My Nefesh is gloomy and sighing - gladden it!
2.
Malbim: I have no Simchah in the world from anything other than You - therefore You should answer me.
2)
What is the meaning of "Nafshi Esa"?
1.
Rashi: I direct my heart.
2.
Radak #1: It is supplication with the entire heart. This is like "Nisa Levaveinu El Kapayim" (Eichah 3:41).
3.
Radak #2: This is like a gift - "va'Yisa Masa'os" (Bereishis 43:34).
4.
Radak #3: This refers to hope - "v'Elav Hu Nosei Es Nafsho" (Devarim 24:15).