Why did he say "Hafachti Mispedi l'Machol Li"?
Radak: I was mourning over my soul, if it will die. You informed me via Your Navi "Gam Hashem He'evir Chatascha Lo Samus" (Shmuel II, 12:13).
Malbim: I see that also You heard my Tefilah for Tefilah for this reason, so I will be able to sing to You. He speaks as if all of Yisrael already gathered to eulogize him, for they thought that he died, and suddenly his Neshamah returned. The gathering began to dance.
What is the meaning of "Patachta [Saki]"?
Rashi: This is like "va'Yfatach Es ha'Gemalim" (Bereishis 24:32; he untied their muzzles).
Radak, Malbim: [Via informing me of Your pardon,] You removed my sackcloth [and my affliction over my sins, and girded me with Simchah in place of it].