I find this all rather sad now.
You cook her meals and she re-cooks them?
You take charge of the kids at the weekend so she has some free time.
You are frightened of talking to her about this because she over-analyses the way you say things (instead of concentrating on the words?)
She shows you no affection whatsoever?
I can't get my head around how someone so passionate could suddenly change - or even gradually as you say, to becoming cold and distant.
Was there anything in the past that could have been a trigger? A death of one of her relatives? A traumatic birth? PND? How was she after she had her children? Were the pregnancies easy? How about the births? Is she a depressive person prone to attacks of low self-esteem?
Another thing that worries me is that you don't seem to know your wife very well at all. It's as though you are both strangers just house-sharing. She has changed beyond all recognition and I'm not sure if there is anything to salvage here.
Why do you think she is attending counselling sessions?
I'm sorry but I'm inclined to go with what others are saying now. It may be time to move on from this marriage. Now I know you will say "but what about the kids" however staying in a loveless marriage is hardly likely to be good for them either is it? If you can keep any split amicable there is no need for them to suffer. But if you stay together like this, where is the teamwork? Who makes decisions about the kids? What about when they hit their teens? Are you both going to be singing from the same songsheet or will your differences stick out like a sore thumb, making life difficult not only for you but your growing teenage children who need you to be a unity. Children learn about relationships from those around them and what they are learning from you two is that there is no warmth, no love and neither does there have to be in a marriage - is that a good lesson?
Thing is, you've said you need to talk to her, but you haven't done so. Which leads me to think that you really are scared of her. It's a no-win situation here and all the advice we can give means not one jot if you are not willing to at least try. We're all second-guessing as to what is going on with your wife.
Personally I would leave the computer on this thread so she can read it.
But I'm really not sure that this marriage is going to work. I think she's unwilling to do anything to change this routine you have which leaves you with two choices, to either go with it until the kids leave home or carve out a better life for yourself and give her the chance to find love too.