I think that what you're feeling is probably surprisingly normal. I don't know if that will make you feel better or not, but you're not the only one who's sat up after a number of years and thought, "what am I doing here?" Sometimes we just find ourselves in a spot where things aren't connecting in our brain.

Others have suggested counseling, and I'd echo it. Couples and individual, if you can afford it. A good counselor can help you talk through things and see things in yourself that you may not have realized were there. Most people here demand a Christian counselor, but I don't think it's necessary. Just make sure they're licensed and actually know what they're doing.

In the end, you can leave, but if your wife still loves you and you theoretically love her (or did at one point) and just can't feel it right now, you can get back there. Every relationship has its ups and downs, and you shouldn't focus on others who still report butterflies and think that it means you're broken for not feeling them. Focus a bit on what makes you feel close to her; if it's physical touch, do that. Obviously if she's ill, that'll be more complicated, and honestly that might be why you're struggling so hard, but do what you can, even if it's just cuddling for now.

As far as staying for the kids, I don't know. I've known people who were messed up by divorce, and I've known people who were messed up by watching their parents stay in a zombie marriage. I think kids are better off when they can see their parents in a loving situation, even if it's with stepparents. But I don't think divorce is your answer right now. Give it time, therapy, and make an effort to connect and figure out both what you liked about her and what you like about her now. You can come back from this, and could easily find yourself in a few years looking back and wondering what you were thinking at this moment. Don't give up yet.