The last clause limits and explains the former, how God answered him so speedily, not by giving him the thing which he desired in that very instant, but by giving him inward support and patience to wait God's time, and to bear all his troubles cheerfully in the mean time, which was a singular mercy, and indeed greater than the actual donation of any temporal blessing.
Related Commentaries of Psalms 138:3
Psalms 138:3
3 In the day when I cried thou answeredst me, and strengthenedst me with strength in my soul.