Well Tyks, by reading your signature snippet (I wish everybody would do this, it does make it easier to remember who is who) you were married 20 years and BD was less than a year ago.
You should be sad and annoyed and very confused. BD is a big deal.
Not knowing what is going on in their heads can drive us crazy. Sure, you can ask, write a long letter, etc etc.... unfortunately it won't do any damn good. Either they ignore, lie, or open up and run again. That gets even more infuriating. Oh wait, my husband will be the exception.... I think we ALL think this one time or another.
I can tell you to stop obsessing about what's going on in his head but, I doubt you will listen. We all gotta do what we gotta do until we can't stand it anymore.
I think you're doing great.