TellMeSo, I'm sorry you're going through this. It's an awful situation.
My H said very similar things at BD and at first I didn't even consider the possibility of infidelity. But I soon read and heard that I should brace myself for it, that it's very common for there to be someone else when a spouse behaves this way. H wasn't cagey with his phone or computer but his mood swings and refusal to work on the marriage and claims to want to be "alone" and "grow as a person" all fit with the wayward spouse script. He was adamant that there was nothing I could do or say to change his mind. And sure enough, when I thought about it a bit more, I soon came up with a suspect for OW, and I learned two weeks ago that I was correct (and also that there is now a OW2). It's horrible to hear even when you prepare yourself for it. But I imagine it would worse if you don't.
It is a really painful thing to even think about, I know. But can you think of anyone W might be talking to? A coworker, someone from her past, a "friend"?