I still thought that it is not we who sin but some other nature that sins within us. It flattered my pride to think that I incurred no guilt and, when I did wrong, not to confess it so that you might bring healing to a soul that had sinned against you. I preferred to excuse myself and blame this unknown thing which was in me but was not part of me. The truth, of course, was that it was all my own self, and my own impiety had divided me against myself. My sin was all the more incurable because I did not think myself a sinner.

[Adhuc enim mihi videbatur non esse nos qui peccamus, sed nescio quam aliam in nobis peccare naturam, et delectabat superbiam meam extra culpam esse et, cum aliquid mali fecissem, non confiteri me fecisse, ut sanares animam meam, quoniam peccabat tibi, sed excusare me amabam et accusare nescio quid aliud quod mecum esset et ego non essem. Verum autem totum ego eram et adversus me impietas mea me diviserat, et id erat peccatum insanabilius, quo me peccatorem non esse arbitrabar, et execrabilis iniquitas, te, deus omnipotens, te in me ad perniciem meam.]

Augustine of Hippo (354-430) Christian church father, philosopher, saint [b. Aurelius Augustinus]

Confessions, Book 5, ch. 10 / ¶ 18 (5.10.18) (c. AD 398) [tr. Pine-Coffin (1961)]
    (Source)

The central phrase about healing a soul that has sinned is from Psalm 41:4.

(Source (Latin)). Alternate translations:

For I still thought "that it was not we that sin, but that I know not what other nature sinned in us"; and it delighted my pride, to be free from blame; and when I had done any evil, not to confess I had done any, that Thou mightest heal my soul because it had sinned against Thee: but I loved to excuse it, and to accuse I know not what other thing, which was with me, but which I was not. But in truth it was wholly I, and mine impiety had divided me against myself: and that sin was the more incurable, whereby I did not judge myself a sinner.
[tr. Pusey (1838)]

For I still thought, "that it was not we that sin, but that I know not what other nature sinned in us;" and it delighted my pride to be free from blame, and when I had done any evil, not to confess I had done any, that Thou mightest heal my soul because it had sinned against Thee: but I loved to excuse it, and to accuse I know not what other thing, which was with me, but which I was not. But in truth it was wholly I, and mine impiety had divided me against myself: and that sin was the more incurable, whereby I did not judge myself a sinner.
[ed. Shedd (1860)]

For it still seemed to me “that it was not we that sin, but that I know not what other nature sinned in us.” And it gratified my pride to be free from blame and, after I had committed any fault, not to acknowledge that I had done any, -- "that Thou mightest heal my soul because it had sinned against Thee;" but I loved to excuse it, and to accuse something else (I wot not what) which was with me, but was not I. But assuredly it was wholly I, and my impiety had divided me against myself; and that sin was all the more incurable in that I did not deem myself a sinner.
[tr. Pilkington (1876)]

For I still believed “that sin was not a voluntary act , but that some other nature, I knew not what, sinned in us;” and it flattered my pride to regard myself as free from fault; and when I had done anything wrong, not to admit that I had done it, “that Thou mightest heal my soul, because it had sinned against Thee;” but I loved to excuse it, and to accuse that which was in me, I knew not what, save that it was not myself. But indeed it was all myself, and my iniquity it was which had divided me against myself; and the sin which led me to deny, that I myself was a sinner, was on that account all the more incurable.
[tr. Hutchings (1890)]

For as yet I held that it is not we who sin, but that some alien nature sins in us; and my pride delighted in the thought that I was not to blame, and that, when I had done evil I need not confess that I had done it, to the end that Thou mightest heal my soul, because I had sinned against Thee. But I loved to excuse my soul, and lay the guilt on something else, which was with me and was not myself. But in truth I was one, and nothing but my iniquity had divided me against myself; and the thought that I was not a sinner was the deadlier part of my sin.
[tr. Bigg (1897)]

For I still held the view that it was not we that sinned, but some other nature sinning in us; and it pleased my pride to be beyond fault, and when I did any evil not to confess that I had done it, that You might heal my soul because it had sinned against You: I very much preferred to excuse myself and accuse some other thing that was in me but was not I. But in truth I was wholly I, it was my impiety that had divided me against myself. My sin was all the more incurable because I thought I was not a sinner.
[tr. Sheed (1943)]

For it still seemed to me “that it is not we who sin, but some other nature sinned in us.” And it gratified my pride to be beyond blame, and when I did anything wrong not to have to confess that I had done wrong -- “that thou mightest heal my soul because it had sinned against thee” -- and I loved to excuse my soul and to accuse something else inside me (I knew not what) but which was not I. But, assuredly, it was I, and it was my impiety that had divided me against myself. That sin then was all the more incurable because I did not deem myself a sinner.
[tr. Outler (1955)]

I still thought that it was not ourselves who sin, but that some sort of different nature within us commits the sin. It gave joy to my pride to be above all guilt, and when I did an evil deed, not to confess that I myself had done it, so that you might heal my soul, since it had sinned against you. I loved to excuse myself, and to accuse I know not what other being that was present with me but yet was not I. But in truth I was the one whole being, and my own impiety had divided me against myself. That sin was the more incurable whereby I judged myself to be no sinner.
[tr. Ryan (1960)]

For I was still of the opinion that it is not we ourselves who sin, but some other nature which is in us; it gratified my pride to think that I was blameless and, if I did something wrong, not to confess that I had done it, so that you might heal my soul, because my soul had sinned against you. Instead I liked to excuse myself and accuse something else -- something that was in me, but was not really I. But in fact I was wholly I and it was my impiety which had divided one me from another me. My sin was all the more incurable because I imagined that I was not a sinner.
[tr. Warner (1963)]

I still believed that it is not we who sin, but some undefined "nature" within us, and to be thus faultless was joy to my pride, as it was not to confess some evil I had done that you might heal my soul when I had sinned in your sight. I loved to excuse myself and blame something else which was with me, but not I. But truly it was wholly I, and my wickedness had divided me against myself. That sin was more incurable in which I did not consider myself a sinner.
[tr. Blaiklock (1983)]