Jonathan, with some interesting comments on one of Shakespeare's sonnets. (He's always thinking up productive & well-defined poetry self-edification projects.)

I would interpret the poem slightly differently, I think (it's complicated!). Yes, narcissism is not self-love per se; it's merely one symptom of it, one form it takes. But the speaker is not "lying", because he's not accusing himself of narcissism : he's accusing himself of overweening self-love. This excess is none other than original sin, which fills "every part"and is "grounded" in his inward heart. When he sees his aging image in the glass, he's reminded of his mortality & human frailty, and recognizes that he's been projecting his inescapably excessive self-love - in narcissistic fashion - on his admired young friend. The glimpse of his aging image in the mirror is not a narcissistic moment, but a moment of truth.

Why is overweening self-love "original sin", inescapable? Well, it's grounded in our "god-like" subjectivity and self-consciousness. I love this sentence from Nicolaus Cusanus : "Omnia enim universalia, generalia atque specialia in te Iuliano iulianizant." [All universals, generalities, and specifics julianize in you, Giuliano.]

We are each a little creative world, a microcosm : a situation which has both its glories & its dangers, which is why we love to - and must - keep looking into (& through) various kinds of "glasses".