Why is David so sad and crying so much?
Radak: I sigh and worry about my illness.
What is the meaning of "Ascheh"?
Rashi: It is an expression of "Sechi u'Ma'os" (Eichah 3:45, i.e. phlegm - Rashi there), "v'Haysah Nivlasam ka'Suchah" (Yeshayah 5:25). I make my bed repulsive via tears.
Rashi citing Menachem: This is like "Ka'asher Yefares ha'Socheh Lishchos" (Yeshayah 25:1, i.e. one who swims) and "Mei Sachu" (Yechezkel 47:5).
Radak: (It is to wash.) The Targum of "va'Yirchatz" (Bereishis 43:31) is v'Aschei. (Or, it is to swim. In either case) this is an exaggeration.
Why did he say specifically at night?
Radak #1: Illness gets more severe at night; a person sighs and cries over it.
Radak #2: Then his household is sleeping, and they do not see him
What is the meaning of "Amseh"?
Rashi: I wet it like water.
Radak: It is [melting,] like "Himsiv Es Lev ha'Am" (Yehoshua 14:8), "va'Temes ka'Ash Chamudo" (below, 39:12). This is an exaggeration.
