The second part of the song passes from prayer to thanksgiving at the prayer being heard.
What shall I say? - the language of one at a loss for words to express his sense of the unexpected deliverance.
both spoken ... and ... done it - (Num 23:19). Both promised and performed (1Th 5:24; Heb 10:23).
himself - No one else could have done it (Psa 98:1).
go softly ... in the bitterness - rather, “on account of the bitterness”; I will behave myself humbly in remembrance of my past sorrow and sickness from which I have been delivered by God’s mercy (see 1Ki 21:27, 1Ki 21:29). In Psa 42:4, the same Hebrew verb expresses the slow and solemn gait of one going up to the house of God; it is found nowhere else, hence Rosenmuller explains it, “I will reverently attend the sacred festivals in the temple”; but this ellipsis would be harsh; rather metaphorically the word is transferred to a calm, solemn, and submissive walk of life.