Thanks for all the sound advice. I should clarify a couple things: There's been no infidelity. The division of labor is not an issue. My wife pulls her weight. The issue is that she invests zero into our relationship. I plan all of our dates, I buy her flowers, I plan our family activities, I make her lunch, I watch movies she wants to watch, I plan birthday celebrations, etc. She is "grateful," but there's no reciprocation. And why would there be? She's learned--despite the arguments, counseling that I demanded, ultimatums--that I will always be there to see to her needs. When it comes to a head a few times a year, she does just enough for a week or two to take the heat off, and then goes back to the same exact behavior. She sits in counseling and says she understands, accepts responsibility, is going to take action, and then forgets it all when she walks out the door. She has no intrinsic motivation to change, because as far as she's concerned, things are great! The only things that she's intrinsically motivated to do are work, which takes up a staggering amount of her time, and spend time with her parents and sisters whenever she has time off. She has never in 9 years planned a trip for just the two of us, or even a trip for just us and the kids.
My plan is about me, not about her. Neglecting my half of the household/kid duties is cutting of my nose to spite my face. For one thing, it will make me less happy, because those are things that I take pride in doing well, and living in a messy space depresses me and drains my energy. Secondly, not pulling my weight allows her to cast me as the bad guy and feel sorry for herself--she's not going to have to face up to things if she can cast herself as the victim. The approach I'm going to take is this: For the foreseeable future, I'm going to act as if we're roommates who share parenting responsibilities. I do my share of housework, continue to participate in my kids' lives, and treat her with the politeness and respect that I'd give to a coworker or roommate. Otherwise, my time is mine and I'm going to spend it by myself or with friends on things that make me feel good, rather than my endless pursuit of her approval and affection. I'm not planning dates, I'm not going to visit her family, I'm not calling or texting except for kids/household business, I'm not cooking special meals, I'm not coming home with flowers, and I'm not arranging dates or joint social events. If this makes an impression on her that causes a long term change, great. If it doesn't, I've already taken major steps to improving my life. Maybe I'm deluded, but I think it's a plan that makes sense in my particular situation.