I had a lot of feelings while reading this book. I came to really love and enjoy Juana's narrative so much that I was dreading the end, knowing how her life was going to turn out. The scene where she was waiting in the wings to confront her power-hungry husband and the Cortes? I was anxious, my heart was pounding for her. When she swept out, all I could think was "yes! good for you, girl." I believe it was in reality her father who had first persuaded the Cortes that she was mentally unstable - but Gortner had to keep that final betrayal for the ending, it was a more dramatic way for the novel to progress.
I am willing to believe that she may have inherited some sort of mood disorder (after all, European monarchies are not known for their willingness to widen their gene pools), but it was definitely exacerbated by her imprisonment. Wouldn't you go crazy too? Locked up in a nunnery first by your father and then your son, having your rights and freedom taken away for the rest of your natural life?