I understand that you feel that way, T. Give yourself some time and ctry to think about WHY it bothers you like it does. Is it just the fact that it happened at all? Is it because you convinced yourself it would never happen? Is it because you waited, whereas if you had known you might have acted differently?
I dunno, it seems like what your W was asking for was forgiveness. Maybe by pointing out that she feels like you didn't take her decision to separate seriously, she's trying to say, "I tried to warn you. The fact that you tried so hard to hold on only makes me feel worse now." But I just keep coming back to the idea that, hey, after she flew off and looked around a bit, she still decided to come back to you.
She might be thinking now that she wants to come back, but not if that means she will be resented by you for the rest of her life.
Give it some time, T, I urge you to go for a run, see friends, go bowling, something to get your mind off it. You need some perspective, but that takes time.
M41 W42 M 12 T 15 S10, D9, twin Ds 3 1/2 BD 1/2/2013 Living as roommates Working on D agreement w. mediator 5/13