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Did your W say she wants a divorce or wants to leave, or just that she's unhappy? Have you asked what she wants to do? Does she want to work on things with you, or does she need space?
It is normal for your marriage to hit a low point after the birth of young children. If you both recognize that, you can get through it.
A few more books for you to read:
"The Sex-Starved Marriage" -- My wife and I both read this and it helped us immensely
"The 5 Love Languages" -- This will *really* help you make sure you're delivering what she needs
"His Needs, Her Needs" -- Great book about keeping love alive in marriage.
I would read them in that order in your sitch. A very important thing for you to know is that you don't have to do anything quickly -- there is no ticking time bomb. "Doing nothing" regarding your marriage is often a fine plan, take the time to work on yourself, do your reading, and give W space to sort out her feelings.
Accuray
Married 18, Together 20, Now Divorced M: 48, W: 50, D: 18, S: 16, D: 12 Bomb Dropped (EA, D): 7/13/11 Start Reconcile: 8/15/11 Bomb Dropped (EA, D): 5/1/2014 (Divorced) In a New Relationship: 3/2015