Not sure how one would know how their ex is doing emotionally other than their words, which we cannot trust.
Well 25years, I tend not to write what I can't provide evidence for!!
There are many many signs (these guys may run fast and hard at first for after a few years many of them begin hanging around the edges of our lives again down the line.)
Since I am not writing War and Peace I won't give you all the details, because I don't have time to write them!!.
However, two quick points. My 3 children (and remember we are now nearly 12 years post bomb drop), have some sort of contact with their father, strongly encouraged by me. There were years when they didn't and now they feel sorry for him. Their consensus is that he isn't a happy man.
I still work with a therapist periodically - this is something I choose to do for a whole range of reasons, so all of you Newcomers, don't worry - therapy this far in isn't necessary or mandatory!
My xh contacts me periodically on the flimsiest of excuses, and yes, I have had apologies, masses of regret, but also a lot of self-justification . . . . anyway, I share these with my therapist and his professional view of the email contacts and their content is that they come from a man who is deeply unself-aware, and very very unhappy. But also extremely manipulative.
So on the evidence, and short of my xh saying he is unhappy, I believe he is. I don't need him to be, and in many ways I wish he were happier
I don't think they run because they are unhappy, but because they are depressed covertly, and/or not getting what they think they want out of life.
Initially they are on a high, or at least many of them act this way, and tell everyone they have never been happier. I can't say how genuine it is, but my xh has told me in the last couple of years that everything I have predicted about him leaving and the consequences were 100% right. He even asked me what was going to happen next. I have a pretty good idea, but didn't think sharing it would help anyone.
He never did marry first OW and his current wife appears to be very like his mother.