DadFMF,
Thats exactly what I said to my husband during one of our 'conversations' sometime ago.
I told him that he "left the marriage" (which he did not appreciate). I explained to him that we were only legally married (as he was cake eating). Emotionally, he had decided to spread his wings, was interested in his own space. That is not a marriage - she's either with you in the HOME (as a wife & mother) or she's not. She cannot have her cake & eat it too.
Your wife has left - maybe not on paper but emotionally she's gone.
I know this from my personal experience. I 'held my ground' when I told my husband
he left ... & in the end he understood exactly what I meant because he knew how he was thinking, what he wanted & what he was doing at the time. My dilemma is unlike yours because of the ongoing 'midlife debate' of which we are part. My husband is over 50. At this time, I await 'my jury' re:
was his behaviour a character flaw or a symptom of midlife crisis? This, I will post on a thread in about week, as I do not want to 'hijack your thread,' with issues that are not specifically your own.
You need to
press on with your own life, your own business & personal affairs.
That can't hurt anyway! 
'Time waits for no one' - You are losing time re: her, time she is
not losing re: you! Time meant for you - time you give of yourself as a father (which will be better if you take care of you - an unhappy, frustrated parent will also impact on the children & not only you)
You mentioned investing in the marriage before ... Invest in yourself. Regardless of how things unfold (with her in future??? without? ), that is one investment that you & your children will benefit from - BIGTIME.