I'm new here myself (found out about 3 mos ago)-- but just wanted to tell you I'm so sorry about what you're going through. Mine basically said/did all of the same things. It's hard to understand and hard to take.
Why they throw away what they have with someone who has loved them as much and as long as we have... for someone they basically just met and hardly know... is beyond me.
The only "advice" I can offer at this stage is that this may just be the tip of the iceberg of what is going on with him (he sounds all over the place), so be prepared for that. And if he's that wimpy and disordered about the whole thing, you may be well-served by taking a major step back and letting him live with his choices.
I know it's all happening very quickly, but you are smart to realize you cannot change him; you can only change the way you react to him.
Get a good attorney, and good luck.
I could have said a lot of what you shared in your post. It doesn't matter when he became "unhappy". Who isn't unhappy once in a while in their lives??? Your WS has extremely poor coping skills and boundaries and lacks courage and integrity. You offered him the GIFT of reconciliation and he turned it down.
If she is his subordinate, I'm sure this will get them in to a lot of trouble. Just saying.
Now you need to focus all your energy on YOU. Ok? Make sure you are drinking enough fluids, eating healthy, continue therapy, see your doctor if you are experiencing difficulty sleeping, depression, anxiety, etc. See an attorney so that you receive what you are entitled to. Focus on taking care of your needs, process all the shitty emotions that are happening so that you can heal and rebuild and move forward. You CAN do this!
P.S. I have a link below that you might want to check out.