A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
psa 71:1The Psalmist, probably in old age, appeals to God for help from his enemies, pleading his past favors, and stating his present need; and, in confidence of a hearing, he promises his grateful thanks and praise. (Psa. 71:1-24)
(Compare Psa 30:1-3).
psa 71:3given commandment--literally, "ordained," as in Psa 44:4; Psa 68:28.
rock . . . fortress-- (Psa 18:2).
psa 71:4cruel man--corrupt and ill-natured--literally, "sour."
psa 71:5trust--place of trust.
psa 71:6His history from early infancy illustrated God's care, and his wonderful deliverances were at once occasions of praise and ground of confidence for the future.
my praise . . . of thee--literally, "in" or "by Thee" (Psa 22:25).
psa 71:10The craft and malicious taunts of his enemies now led him to call for aid (compare the terms used, Sa2 17:12; Psa 3:2; Psa 7:2).
psa 71:12(Compare Psa 22:19; Psa 40:4).
psa 71:13(Compare Psa 35:4; Psa 40:14).
psa 71:14The ruin of his enemies, as illustrating God's faithfulness, is his deliverance, and a reason for future confidence.
psa 71:15for I know . . . thereof--innumerable, as he had not time to count them.
psa 71:16in the strength--or, relying on it.
thy righteousness--or, faithful performance of promises to the pious (Psa 7:17; Psa 31:1).
psa 71:17Past experience again encourages.
taught me, &c.--by providential dealings.
psa 71:19is very high--distinguished (Psa 36:5; Isa 55:9).
psa 71:20depths of the earth--debased, low condition.
psa 71:21increase, &c.--that is, the great things done for me (Psa 71:19; compare Psa 40:5).
psa 71:22To the occasion of praise he now adds the promise to render it.
will . . . praise--literally, "will thank."
even thy truth--as to Thy truth or faithfulness.