sorry for your loss, I can't guess why he left but I take it the kids are grown and mostly out of the way.Speaking for me I left or made my wife leave and I am not going thru any midlife crisis. She was just a bad wife and mother. My kids are in upper teen years and she just wanted to be a friend instead of a mother to them.
lonely yes, scared yes gonna survive yes, I have not told her why.. I have to go thru my lawyer before I could do that. I will be fair honest and as open as allowed but ...
was there any hint? signs how is his mental state. money problems ? guilt or were you on him about anything or not (sexually close)
we can guess for years but your husband is the only one who can answer this. continue on with your life and try to find something you enjoy doing and you will make out ok. best of luck to you...