I told my current T not to leave me. I have a fear of abandonment because my parents divorced when I was three and my brother left to live with my dad during my elementary school years.
I remember my first psychologist and how he abrubtly terminated sessions. I didn't seem to miss him much. But, when my second T left the office. I had a major struggle with a strong feeling of abandonment. My T says that my friends will stay in my life. Do you have family or friends who will stay in your life? I like to tell myself that I will not leave me. I know this sounds weird but I like to talk to myself like I am a child at times. I would definately say that it is okey to mourn the lose of a T. I would mourn the lose of my current T a lot if she left me. I would probably write some therepeutic letters to her about how I miss her. I used this idea when my Great aunt and grandma died. I think it could work for you too. Also you have probably internalized a lot of what T says and looks like. I suspect that you can close your eyes and see him or her. I hope you find a way to mourn the lose of a very close and intimate relationship.