There are several ways in which the Sibyl was changed. First, according to the testimony of the Sibyl, when she was selected to be a priestess of Apollo, she was a innocent virgin. In fact, she one of the few. After she became a priestess of Apollo, her sexual purity was no more. She became a vessel of the god Apollo. Moreover, at first she liked the privilege of being a priestess
Second, in time she realized that she was being used by the priests of Delphi. She also realized that she was being used by the god. Hence, at this point she lost all innocence and she realized that she lived in a broken world.
Third, she falls in love with childhood friend and seeks to leave her post, but she is brought back and her lover eventually dies. She sees this as the revenge of the god.
Finally, she conceives a child and she realizes that it is the god's son. So, she become a mother, but in the end the son disappears and in an odd twist of fate, she experiences peace.
In conclusion, from an innocent girl, she loses her innocence in every way, becomes a mother, experiences abandonment, and finally experiences an odd peace.