Why did he say "Hineh l'Shalom Mar Li Mar"?
Rashi citing Brachos 10b: Chizkiyah requested (that Hashem save Yerushalayim) in his own merit -- "Zechor Na Asher His'halachti Lefanecha" It was granted in others' merit -- "v'Ganosi Al ha'Ir... Lema'ani u'Lema'an David Avdi." Even when Hashem saved Yerushalayim, it was bitter for Chizkiyah (for it was not attributed to him).
Rashi: Even when I heard about Shalom, that Hashem will save me from Sancheriv, I did not rejoice, for I was about to die.
Radak: I was bitter in my illness about my lack of Shalom.
Radak citing Ibn Ezra: When I [should have had] Shalom, in the middle of my days - he was 39 when he became ill [it was bitter for me]. If the red bile overpowers on a person, he will always be ill in his youth and b'Shalom in his old age. The opposite applies if the Lechah overpowers. The middle years are of Shalom.
Malbim: The first "Mar" is based on Hemir (switching). Hashem switched for me Shalom in place of bitterness. He promised me more years, and also salvation from Sancheriv.
Why did he say "v'Atah Chashakta Nafshi mi'Shachas Bli"?
Radak: You desired my soul to save it from Shachas ha'Balim, i.e. the grave, in which a person's body is Balah (decays).
Why did he say "Ki Hishlachta
Radak: Even though I was liable according to my sins, You cast them "Acharei Gavecha" - this is an anthropomorphism for a man who casts a matter in back of him, so no one will see it. Malbim - "Al Kol Pesha'im Techaseh Ahavah" (Mishlei 26:26).
