I'm not familiar with your sitch, how long were you together?
The common thing in all of our situations is that for the woman, they've been in pain for a lot longer than we realized. Most women don't want to walk away from a marriage and hang on as long as they can until they get to their breaking point, and by then its usually too late.
In the past, did y'all fight over something and then you'd change for a short while, and then things got back to the same? It's a common cycle.
You may FINALLY be doing the changes your W has been wanting for a long long time, so it's difficult for her for two reasons: She wonders if they changes are true and ever lasting, and she feels that now that she's leaving you, you want to better yourself for the next woman since you never changed for her.
Me: 43 W: 37 Together: 18 M: 15 D: 8 yrs old ILYBNILWY: March 2011 She Filed for D: August 2011 She moved out: Sept 1, 2011 Reconciled: May 2012 Divorce Case dropped: July 2012