The “greatest Commandment” is to love God with one’s whole being. Our love for others, including for ourselves, must be “in God” - within, that is, our primary love for God. To see others (including ourselves) with the “eyes” and “heart” of God is our calling. Thus, to mind one’s own business as a Christian is to be very concerned about “what other persons are doing in the world or spiritually”, because God does direct us to be concerned. We are our brother’s keeper, contrary to the mind of Cain, who killed his brother Abel out of jealousy.
Righteous love for others, and the desire to help them on the path to their own happiness, is no easy task. In most cases, the best way to love them is to pray for them to God, and to Mary their mother, that His holy will for their happiness and salvation be accomplished. And include in that prayer, your own intervention - if God would show you what His will is for you, in their lives.
To live in rightful knowledge of oneself - IMHO - is to courageously insist on Truth concerning oneself, as well as Truth concerning God. The opposite of that insistence is very common, and very popular! People love to tailor a god to fit their lifestyle, and to fabricate a self-image to suit their ego. Fiction is more popular than Truth, and thus the great danger of idolatries and of self-deceit. Rightful knowledge of God and rightful self-knowledge go together - to seek one is to seek the other; to avoid one is to avoid the other. Thus the first step in the journey to God is repentance: the denial of self-love for the sake of truthful pursuit of and love for God. In Him - denying ourselves - in Him we can find ourselves.