Who says this?
Rashi: Ru'ach ha'Kodesh replies so.
Radak: David says so about himself and every Tzadik.
Malbim: Achitofel said so to David (do not worry about rebellion...)
What is "Yehavcha"?
Rashi (from Megilah 18a): Your load.
Radak: Yehav is like Nasan (gave). Cast your needs on Him, what He gave to you until today, and He will finance you.
What is the meaning of "Yechalkelecha"?
Rashi: He will bear your burden. This is like "ha'Shamayim u'Shmei Shamayim Lo Yechalkelucha" (Melachim I, 8:27). The Targum of Kilkul is Mesuvar, like Targum Yonason (of "Mechalkel" - Mal'achi 3:2).
What is "Mot"?
Rashi: It is the foot slipping.
QUESTIONS ON RASHI
Rashi writes that "Yehavcha" is your load. How does one cast his load on Hashem?
Radak: Do not fear people. Trust in Hashem, and He will support you.