St. Augustin on the Psalms.
136. Thus, then, as if giving a reason why he had cause to weep much, and to mourn deeply for his sin, he saith, "Righteous art Thou, O Lord, and true is Thy judgment" (ver.137). "Thou hast commanded Thy testimonies, righteousness, and Thy truth exceedingly" (ver.138). This righteousness of God and righteous judgment and truth, is to be feared by every sinner: for thereby all who are condemned are condemned of God; nor is there one who can righteously complain against the righteous God of his own damnation. Therefore the tears of the penitent are needful; since if his impenitent heart were condemned, he would be most justly condemned. He indeed calleth the testimonies of God righteousness: for He proveth himself righteous by giving righteous commandments. And this is truth also, that God may become known by such testimonies.
137. But what is it that followeth: "My zeal hath caused me to pine" (ver.139); or, as other copies read, Thy zeal? Others have also, "The zeal of Thy house:" and, "hath eaten me up," instead of, "hath caused me to pine." This, as it seems to me, has been considered as an emendation to be introduced from another Psalm, where it is written, "The zeal of Thy house hath eaten me up:"  a text quoted also, as we know, in the Gospel. The two words, however, "hath caused me to pine," and "hath eaten me up," are somewhat like. But the words, "my zeal," which most of the copies read, occasion no dispute: for what wonder is it if every man pineth away from his own zeal? The words read in other copies, "Thy zeal," signify a man zealous for God, not for himself: but there is no difficulty in using "my" in the same sense...The Psalmist's jealousy is therefore also to be understood in a good sense: for he addeth the cause, and saith, "Because mine enemies have forgotten Thy words."...
138. Then considering with himself with what a flame of love he burned for the commandments of God: "Fiery," saith he, "is Thy word exceedingly, and Thy servant hath loved it" (ver.140). Justly jealous was he of the impenitent heart in His enemies, who had forgotten God's word; for he endeavoured to bring them unto that which he himself most ardently loved.
139. "I am young, and of no reputation; yet do I not forget Thy righteousnesses:" not as my enemies, who "have forgotten Thy words" (ver.141). The younger seems to grieve for those older than himself who had forgotten the righteousnesses of God, while he himself had not forgotten. For what meaneth, "I am young, yet do I not forget"? save this, Those older than me have forgotten. For the Greek word is neoteros, the same as that used in the words above, "Wherewithal shall a young man cleanse his way?"  This is a comparative, and is therefore well understood in its relation to some one older. Let us therefore here recognize the two nations, who were striving even in Rebecca's womb; when it was said to her, not from works, but of Him that calleth, "The elder shall serve the younger."  But the younger saith here that he is of no reputation: for this reason he hath become greater: since "behold, they that were first are last, and they that were last first." 
140. It is no wonder that they have forgotten the words of God, who have chosen to set up their own righteousness, ignorant of the righteousness of God;  but he, the younger, hath not forgotten, for he hath not wished to have a righteousness of his own, but that of God, of which he now also saith, "Thy righteousness is an everlasting righteousness, and Thy law is the truth" (ver.142). For how is not the law truth, through which came the knowledge of sin, and that which giveth testimony of the righteousness of God? For thus the Apostle saith: "The righteousness of God is manifested, being witnessed by the Law and the Prophets." 
141. On account of this law the younger suffered persecution from the elder, so that the younger saith what followeth: "Trouble and hardship have taken hold upon me: yet is my meditation in Thy commandments" (ver.143). Let them rage, let them persecute; as long as the commandments of God be not abandoned, and, after those commandments, let even those who rage be loved.
142. "Thy testimonies are righteousness unto everlasting: O grant me understanding, and I shall live" (ver.144). This younger one prayeth for understanding; which if he had not, he would not be "wiser than the aged;"  but he prayeth for it in trouble and hardships, that he may thereby understand how contemptible is all that his persecuting enemies can take from him, by whom he saith he hath been despised. Therefore he hath said, "and I shall live:" because if trouble and heaviness reached such a pitch, that his life should be terminated by the hands of his persecuting enemies, he will live for ever, who preferreth to temporal things, righteousness which remaineth for evermore. This righteousness in trouble and hardship are the Martyria Dei, that is, the testimonies of God, for which Martyrs have been crowned.