Could it be possible that she learnt something that was too much for her to handle at such a young age that it rattled her so much that it drove her to insanity? Could it be her questioning her faith and realising how "fake" (can't think of a better word) it was, making her lose a sense of self or feeling alone. Because maybe her madness was caused by a sudden depressive episode. (It would make sense with her image that she constantly maintains to a point that Shallan becomes jealous.)
I don't know if I remember it clearly but she says something about people talking about her behind her back because she was a heretic. Implying she was young when she decided to forgo Vorinism. In the spoiler tag, I have written the moment I'm thinking of when she says this.