OP almost word for words the exact same thing happened to me. Wonderful DH, wonderful marriage, wonderful Dad. I got a text message to say he had got his own place and left and he would not even speak to me.
I can understand how you feel right now. It is such a shock it feels like you have just been told he is dead. I remember it like it was yesterday.
I cannot speak for what might be going on here, but there are really two explanations.
The first is another woman.
The second is that he has been unhappy for some time and has not told you about it.
Right now, you don't know and may not know for a long time. This is the hard part.
In my case, DH had been depressed for some time, hidden it and finally exploded when he could not cope. He describes now that in his mind it was either leave and abdicate all responsibilities so he could care only for himself or suicide. It took me months to discover this (as he was in denial of being ill) and many more months after that to ever get an apology or any compassion. I was left for dead.
I DID NOT know he was depressed, so make no mistake that people can be deeply unhappy, unwell or be having affairs and even in a close marriage if they want to conceal it they can.
Please keep posting here every day to chat about what is going on. It's devastating to go through this. I can't describe the gut wrenching agony of being abandoned so cruelly by someone you love but you can and will survive it.
The shock lasts a long time in my experience. Even now more than a year onwards I feel disbelief. And only now is DH on the phone crying and regretting what he did.
People sometimes get so wrapped up in their own heads that they do things that are nonsensical. What kind of man was our DH before this? Was he a god talker if he was angry or worried? Does he have a history of cheating or deception? Start trying to unravel, as in cases like this you often have to find the answers all by yourself and it is incredibly difficult.
My DH was also a good father and absolutely devoted to me (he still is and never got another woman) but he was capable of behaving like that when he felt it suited him. People are capable of so many unexpected things when you put them in the right situation to do it!