As painful as it might be, I'd use your therapist to help you process it all. Use her while you can. Is she open to doing phone consults after she leaves? I know it's not the same, but that could help you transition. Does she have a professional that she really loves to refer you to?

I do relate to feeling like you need to detach in order to protect yourself. For me, I am not sure that it is a defense mechanism that really works. I end up feeling regret, like I lost important time and still felt awful. It's a painful situation that brings up many old feelings. You can tell her what is going on - what do you have to lose?