I've had an epiphany. I don't have any control over this situation, and I find that thought ... liberating. Let me elaborate.
This morning, a good friend of mine observed: "I think a lot of your trepidation with her leaving has more to do with your own feelings on your ability to be in control of your life." While I don't know that I necessarily agree 100% with that statement, it does have elements of truth. Part of this may indeed be a control thing for me. But, it's a funny thing about control, sometimes not being in control can be a good thing too.
When I was promoted to Sergeant in the Army, one of the first things I said was "You know, there's a lot to be said for being a Private." My reasoning was that as a Private, you really don't have any problems. If a problem comes up, you tell your Sergeant and now it's his problem. As a Sergeant, however, not only do you have to deal with all your own problems, you also have to take care of your subordinates and look out for their well-being.
In this situation, I don't have any control. That also means I cannot be held accountable -- especially to myself. I have done everything I can, and I can do no more. It's time to focus on things I can control.