Found my mom's inappropriate messages to her ex

Hello everyone, I (19 M) went through my mom’s phone recently and saw that from 2016 -2018 she was emailing her ex boyfriend from college. In the email they exchanged twisted sexual fantasies and stories as well as a few nude photos. Finding this out was a big surprise for me, as I have grown up in a loving household and my parents have always been close. As far as I know, her ex lives in Europe now and she never met up with him or talked with him beyond email after they broke up in 2004.

I also went through her notes app and saw from 2016 - 2022 she was making notes almost daily about how she made the right choice of choosing my dad over her ex. She even made a pros and cons list about the attributes my dad and her ex have. She kept bringing up the fact that her ex chose to be with a different woman and didn’t give my mom enough attention when they were together. She also said that she is satisfied with her current life as she is with a rich loving husband, while she gets to choose whether she wants to give attention to her desperate ex.

The part that mainly confused me after hearing this was why she continued to send messages to her ex. She seemed happy with being with my dad, wrote about how her kids bring her joy, and made it clear that her ex treated her terribly. Still, she chose to willingly engage in emails with him, even after deciding her ex was a bad person. She also constantly wrote about how she made the right choice, making me wonder if she is saying this just to cope or if she really believes it.

I am now wondering if I should tell anyone about the incident. I am considering telling my dad or confronting my mom, but I am worried that this is unnecessary and I can break the family apart. It seems like she is longer in contact with her ex because her last email said “bye forever and have a nice life”. She also stopped sexually romanticizing about him after her last email, and stopped mentioning him in her notes in 2022. Maybe she finally closed this chapter in her life, and if we were to bring it up I would only re-ignite it. Do you think it would be beneficial for me to tell someone in my family about it, or should I try and forget about it and move on with my life? I also cannot view my mom in the same way after learning about her disgusting sexual experiences, including stories about having sex with other women. Does anybody have any tips for forgetting or coping with this?

TL;DR My mom sent inappropriate messages to her ex while with she was with my dad 7 years ago.