isaiah 38:15 What shall I say? he hath both spoken unto me, and himself hath done it: I shall go softly all my years in the bitterness of my soul. - Free Bible Online

      15-20. 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-- (Nu 23:19). Both promised and performed (1Th 5:24; Heb 10:23).
      himself--No one else could have done it (Ps 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, 29). In Ps 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.

JFB.