@Germlat,
That's okay. Your sentiments are part of reality of what fuels people to conceal the truth. My response to you is important so as to show the Original Poster how to not let those comments retract someone back into their shell.
She didn't have to tell us she cheated in the past, but she did. I believe that confession is due to her growth as a person to manifest externally what she desires to be internally. People are special for the power to change their identity by repetitive action.
As it sits now, you don't trust her ability to be faithful despite her declaration opposing that assessment. This is likely one of the thoughts racing around the new guys mind, for which she knows and is why she shed so many tears telling him. Jim Rohn states that disgust, although a negative feeling, has the power to create change in a person's life. She doesn't like how her previous actions are straining her ability to achieve the happiness she wants. This creates motivation and a stronger determination to be strong in her wills. She will likely work double overtime ensuring she changes that negative aspect about her current character.
Believe it or not, the best drug counselors who are often always found at drug rehabilitation centers are previous drug abusers. It seems, sometimes those who were making mistakes in something are able to turn around and be more faithful towards a goal than those who never made the mistake.
If it were narcissistic, how do you explain the contradiction of her admission of wrong? I've never heard a narcissistic person apologize or cry tears for doing something wrong.