Helped when I read this last night in Lewis' Collected Letters, in a note to Walter Hooper, Nov 30, 1954, who would eventually edit this very collection of correspondence:
We should, I believe, distrust states of mind which turn our attention upon ourselves. Even at our sins we should look no longer than is necessary to know and to repent them: and our virtues or progress (if any) are certainly a dangerous object of contemplation. When the sun is vertically above a man he casts no shadow: similarly when we have come to the Divine meridian our spiritual shadow (that is, our consciousness of self) will vanish.
--Collected Letters of C. S. Lewis: Narnia, Cambridge, and Joy 1950-1963, p. 535