some of the posts in "my wife is having a midlife crisis." I am a Navy man with 23 yrs of service and 19 years of marriage that has come to a sudden and abrupt ending - by her own doing. She is, and has, done some of the exact same things you have mentioned in your post.
She drops off the youngest at her parents - or leaves him home alone - spends "some" days with her new man, but not all, and claims she loves him.
It would be good for you to visit the thread I mentioned and read through some of the "symptoms" others have witnessed and how they have delt with their situations.
I have learned that you can't control everything that happens in your life, nor can you understand why it happens. Don't jump to conclusions and rash decisions, but also seek some legal advice. I would start with base legal and move on from there.
I also advise that if you haven't seeked counseling, do that first. That is one option I WAS NOT given when my wife told me it was over.

Good Luck and God Bless,
Jim