I agree with Puppy. It sounds like she has a relationship or at least feelings for this OM and she wants to separate to pursue him free from you.
When my wife first dropped the bomb I did everything that you're doing. I addressed all of her complaints about the marriage and become a model husband. But it didn't make a dent in my W. She still moved out with a "too little, too late attitude." A couple months later I discovered that she had been having an affair the entire time.
When people are high on the affair drug it doesn't matter what you do. All they want is their fix, and nothing you say or do will make a difference. The only thing you can do is get some intel to see if she's cheating on you. Check her cell phone records, put a keylogger on her computer, check her email. Is this snooping? You bet. But you need to know if she's having an affair or not because it changes the way you approach this.
Me: 33 WAW/MLC: 33 M: 4+, T: 10+ Separated: Nov 08 A#1: Oct 08 - Jan 09 (exposed and ended) A#2: Feb 09 - ? 1: http://tinyurl.com/mrmistakes 2: http://tinyurl.com/ckch9t 3: http://tinyurl.com/stillwaters3