Albert Barnes Commentary Psalms 143:3

"For the enemy hath persecuted my soul; He hath smitten my life down to the ground: He hath made me to dwell in dark places, as those that have been long dead." — Psalms 143:3 (ASV)

For the enemy hath persecuted my soul — Has persecuted me; has sought my life.

He hath smitten my life down to the ground — He has, as it were, trampled me down to the earth. The word rendered “smitten” means to break in pieces, to beat small, to crush. See (Psalms 72:4; Psalms 89:10; Job 6:9). His very life seemed to be crushed out as one that is trodden down to the ground.

He hath made me to dwell in darkness — He has made my life like that of one who dwells in darkness; he has made it a life of sorrow, so that I have no comfort—no light.

As those that have been long dead — A similar expression occurs in (Lamentations 3:6): He hath set me in dark places, as they that be dead of old. The same Hebrew words are used. The word rendered “long” means age, duration, eternity: (Psalms 139:24). The idea here is that his condition was like that of those who had been long in their graves, who had long since ceased to see any light, whose abode was utter and absolute gloom.