Book VIII of this classic text tells the story of St. Augustine's conversion, and how he managed to conquer any remnants of internal resistance to embracing Christianity and following Christ. Initially, Augustine has to overcome a few remaining intellectual barriers he has against Christianity, but after he has satisfied these quibbles, he recognises that the last barrier that needs to be breached is one that is profundly internal. He speaks of having a second will within himself that rages against his primary will, that desires to do good:

My two wills, one carnal, one spiritual, were in conflict with one and other.

This second will, the carnal will, causes Augustine initially to remain more attached to material things and hedonistic pleasures, even though he is able to reflect in retrospect that these habits did not belong to him, that they were "no more I." The quote in this question comes from this section, where Augustine identifies the profound internal conflict that he recognises going on within him between these two wills; a conflict that is "about myself against myself." In this conflict, he sees his good side that wants to embrace Christianity pitted against his sinful, pleasure-seeking side.