She is transfixed on only remembering the "Good Things" about this ex and not all of the garbage he put her thru. This happens a lot and can even be elevated to become a "Fantasy" as it appears she has done with this ex. Sadly, you can't do much about it, especially without showing you read her journal. Things usually crash and burn when people allow "Fantasies" to become reality. It may also mean the end of your marriage as well if it ever gets that far.

Basically, all you can do at this point is to live your best life with her. Make sure you do all the things a "Good Husband" should do to and for his wife and if she starts to "drift away" nip it in the bud immediately and find out what the problem is. Re-evaluating where your relationship is at this time might be a good thing to do as well plus possibly setting up some visits to a marriage counselor.

I probably would keep an eye out as to where this Ex is living and what he is doing as well since you don't want him to pop into the picture as that will make your life and relationship very difficult. Then again, depending on the situation with your marriage at this point, you may or may not want to invest any additional time and effort into it as you might not be happy with how things are right now, and perhaps neither is she. You have learned about some information you shouldn't have but that you cannot ignore.

I would also stay away from her journal unless she gives you a serious reason (disappearing for hours a day or night with no legit reason) or her permission to read it.

Since you have read it, you should be cautious but don't obsess over it. Not obsessing over it is going to be the really hard part but maybe that is karma's way of teaching you not to read someone else's journal.