Pigpen, you will have to look at WHY she's this way.
If she was a slob, poor eater, messy etc when you met her, there's no reason to expect that she would change. You married a lazy woman and she's still lazy. It's not going to change. I guess you could try marriage counselling, but it's very hard for a leopard to change it's spots.
However, if her housekeeping is on a downward trajectory, I'm wondering if she's dealing with depression (or other mental illness). The lack of motivation to cook and do the domestic things you describe could easily be due to a psychological issue.
Perhaps she's resentful about being a stay at home mom and would rather work? If so, perhaps she'd better off working and having a housekeeper.
Anyway, if things have gotten worse progressively, you need to ask her about it. When you do ask, you need to avoid the question of "why?" As in "why didn't you do X?" Instead, talk about how she's feeling about motherhood and domestics. Thing is, it's often very hard to know why we do something - but it's much easier to identify how we feel about it. If she tells you how she feels (overwhelmed/bored/isolated/depressed/etc/etc) DON'T argue with how she feels. Her feelings are legitimate even if you don't agree or understand. Acknowledge how she feels and use those feelings to try and figure out a way forward. Those feelings will give you a clue as to what kind of help to seek.
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Hmm...I fired off that answer too quickly.
Some more questions:
When you speak about this with her, what do you say? And what does she say back to you?
When you get home from work, what do you do during the evening?
Does she know how to cook healthy meals? What was her role modelling growing up?
Also, I noticed that you mentioned about the dishes being left unwashed for days. So it would seem to me that you're not at all pro active about saying "hey, let's go and do the dishes together". Now, I'm also a stay at home mum too, but my husband would always come and help me clear the kitchen. It's a rotten job and is much more pleasant if we do it together. Likewise, if he's in the room when I'm making the bed, he will always help me. If he's watching the cricket on TV, he might offer to do a load of ironing for me. Or if I'm making a meal our son won't eat, he may cook for our son so that I don't have to do two meals. In short, how do you help out at home?
Also, I'm guessing your sex life is non-existent. Would I be right?
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.