OP, first of all 
Second of all, like the other (
for all of you too) my DH also did this to me. I also thought we were very happy and within a week he was gone, as was my home and life as I knew it. I know how you feel. I think for me that simple sentence would have meant a lot to me when I was going through it as I felt totall lost and alone.
At the time, my DH was also under tremendous pressure and I felt he had some sort of mental health problems. It was so strange as he'd never raised even a niggle with me and all of a sudden it was hopeless. I could never understand how we got from to B.
Turned out in my case he in fact did have mental health problems, and for whatever reason this made him lose his love for me that had always been so strong. He isolated into himself. He blamed me subtly for the fact that he was unhappy, although he had no clear reason why. I think he just felt "I am not happy anymore" and thus thought it must be something lacking in his marriage.
It took years for him to realise he was wrong and was only ill.
Yes, he did eventually realise he was ill and got treatment, but no, at the time he absolutely thought his only problem was his marriage (although like your DH he lacked clear reasons) and refused to get counselling or try anything. It was only when he got considerable more ill after leaving that he got treatment - and even then he did not come home.
No, we were never able to work it out because he never got better. I think I was the love of his life and when he left me and lost me I don't think he ever got over it. He is still in treatment years later for major depression that won't lift. He was classified as having a breakdown, but not until it was too late.
At the time he was just convinced he didn't love me anymore.
Ironically now he is congnitively aware he never stopped loving me, but the very nature of his serious depression makes him unable to sort the problem, unable to care enough to do anything about it, and unable to feel enough hope and positivity.
After years of waiting, I had to move on.
He never met anyone else and lives alone.
It's tragic.
Depressed people can sometimes genuinely lose the ability to feel love, to feel hope, to feel affection and they feel the need to isolate or mke changes and their behavior can be irrational.
That said, the cript is also the same if he has another woman.
At the end of the day the result is the same, and all power is gone from you. This journey you will have to take will be the hardest of your life and you will feel alone and it will feel like he died. You will feel shock, panic, anger, agonising pain and terrible lonliness and yearning and grief.
I am so, so sorry OP. It is so very unfair.
The truth is even if he is able to get to the doctor or the counsellor, if he is unable to talk to you or be self aware enough it does little good. For example he will go to the counsellor and say his marriage i the problem, and he / she will believe it and will help him on leaving the marriage - both unaware the marriage might not be the problem at all.
He will need to take this journey alone now.
I can't express how sorry I am, but you need to love yourself now. That's very important. You can do this.
You are braver and stronger than you think.
xxxxx