He is really running away from himself and his battle is his unhappiness with himself. It has been brewing in him for a while and it can happen once a man gets into or is approaching his fifties. This other woman probably just aided him in his quest to change his life and he may not actually love her. Actually, men go through hormonal changes like women do, their production of testaterone drops and they do begin to feel differently. Hormone level changes can really have an affect on not just women but also men. They are experiencing a kind of withdrawl that often they themselves don't quite understand. So, they have no idea how to articulate it. It's a menopause for men. And as with women, some are impacted more than others.
Truth is, the affects of hormonal changes are still being studied in both females and males. Some men begin to feel depressed and not themselves, and it can actually "scare and confuse them". And while he may be living in the same complex as this woman, it doesn't mean "she" is the true answer and he may STILL be very confused. Biological changes DO have an affect of all of us, some worse than others. It's an actual "chemical change" that affects how people "feel" about themselves and life in general. And we all do change "physically" as well as we begin to even lose our muscle mass.
He has tried to even say to you "it's not you IT'S ME" and that's probably the "truth" even though this has been so confusing for you.
My older brother has been battling prostrate cancer. The surgery and the treatments drastically affected his hormone levels. We were in the waiting room waiting for a hearing as we have been dealing with the loss of our parents and how horrible my sister has been. I observed my husband go over and talk to the three women at the desk and then he shared how one thing he now respects in how women go through menopaus. He talked about his treatments for prostrate cancer and how that's affected his hormone level and he happened to take his grandchildren to a disney movie and next thing he knew he could not stop weeping. These women were VERY moved by what he shared with them, what it can mean to a man facing significant hormonal changes. Most men would NEVER sit and talk about that and I have to say I was very surprised that my brother did share that. Oh, men notoriously have a hard time sharing their emotions. They are typically raised to stay quiet and JUST man up.
There is a scene in the movie "Moon Struck" where the mother is having dinner by herself in a restaurant and she sees a man suddenly have the younger woman he is with blow up and the woman throws her wine on him and walks out. The mother ends up having a conversation with the man who sits and shares with her how he has grown bored with himself and then once in a while a young face looks at him like he is amazing and important and he dates her, BUT it never lasts because they finally realize he isn't ALL THAT and instead just a boring college professor. What was good about that scene is what this man was sharing about himself and what seems to be missing in himself that he has been struggling with.
Truth is, people often go through changes we don't understand and THEY themselves don't understand. I don't think he is deserting you the way you think he is. I think your husband is lost and confused more than anything else.
What I am posting to you here isn’t about making excuses for your husbands choices either. You certainly did not deserve to suddenly have this happen. I just wanted to share some things you probably have not considered yet and I feel these things are important.