I'm so sorry. I know you're heart is broken. I know.
There's nothing you can do but go with it. You can tell her that you know she wants to leave you but you love her and wish she would reconsider and work on the marriage instead.
This is all you can do. If you make her feel guilty, she may stay just for the guilt. This could make it miserable in the long run for the both of you.
You can ask her to come to therapy with you, which will probably be something the LCSW will recommend when you see her. This will help her find out if there is any hope for reconciliation of the marriage.
Again, I am so sorry. This hurts and will hurt. My advice is to not deny yourself the right to grieve this because you are entitled to feel every emotion you have from this - the rage, the sadness, the confusion - everything. Go with it... and see what happens.
The more you fight... the more she runs... the more she runs... the more you suffer. Just be there if she wants to work on the marriage, ask her to come to therapy with you to work on the marriage, and give her the time she needs to make sure that this is it.
I'm hoping that she comes out of the "fog" and decides to save the marriage with you... I'll be checking back for updates.