Without having all the details I cannot say for certain that this is what's going on...
But, I can related (as a woman) to what you are saying.
As a woman, we tend to give everything that we have to a relationship - more than we should and are truly able to give. And then one day we wake and realize that we have nothing left to give. :-(
It's not your fault and it's not her fault. Women just don't naturally no when to say 'no' - especially to someone that they love. They end up giving all of the energy and power to their partner - and in turn blame their partner because they're so exhausted.
Dan and I went through this recently in our relationship (a few years ago). I was ready to leave because I felt that I had given and given and given to the relationship and gotten nothing in return.
What I learned, was that I had to learn to set boundaries and to recognize when I needed a break. This may or may not be what you are experiencing with your wife.
My suggestion is to get this book today (2 copies - one for each of you) and read it.
Men Are from Mars, Women Are from Venus: The Classic Guide to Understanding the Opposite Sex
This is what our good friend and mentor recommended to us and I think that this simple little book saved our relationship.
Oh, and give her the space that she's asking for...