There's nothing wrong about calling for an initial single time session for processing. I don't know whether you terminated too early or not. I do know grieving for a past friend who you will never see again is hard. Even, if you know they are not dead. You just can't see them again. Perhaps, you had a very deep bond with your therapist. Unlike friendships and, say, moving you can't call them regularly just to chat and check up. If you saw T as a friend or greater your grieving could just be normal.