[x]"We are defined, not by our pasts, but by our choices and our sacrifices"
...because who she was is insight into who she is now, and it gives a fuller picture, but it doesn't really change much. She has undergone so many state changes, each at least partially the result of some choice that she herself made, and each of them called for her to sacrifice something that would never come again. It may be that her past returning to her will have a major effect on who and what she is - but if so, it is only because it causes her to choose again, and sacrifice again... and regardless of her choices here, she will never again be who she was.
Of course, this is basically true for *all* mages, but that's kind of what makes it a lesson, no? It's just that she's rather had her face rubbed in it recently. Also, it seems kind of fitting for the insight that brings her to finally let go of The Lie.
Also, it seems better to offer a new lesson, rather than rehashing one of the ones handed to us before.
edit: In retrospect, this one is at best refurbished from stuff that
@MJ12 Commando handed out before, but it's still an upgrade, right?