1)
Why does it say "Ki"?
1.
Radak: His trust and Simchah is only in You, but not in abundance.
2)
Why does he trust in Hashem?
1.
Malbim: This is like he said - "u'Devarecha Yihyu Emes" (Shmuel II, 7:28).
3)
Why does it say "Chesed Elyon"?
1.
Radak: Everything that Hashem does for His creations is Chesed. He is Elyon (over everything), and has ability to save or benefit whom He wants.
4)
What will not falter?
1.
Rashi: He trusts that the supreme Chesed will not falter. Malbim - I explained (in Shmuel II, Perek 7) that Kisei David was founded based on Chesed Hashem, so it will never cease.
