I’m very attached to my therapist. I also have a history of leaving things I care deeply about. To enter the subject more deeply with him could certainly improve the work, not damage it, so perhaps I’m the one pulling away, preferring not to risk any further attachment, on the principle that the more attached you are the greater the pain of the ‘certain’ loss.

Big insight here. It sounds like your T is comfortable with transference, so if you can talk about your history of pulling away, and that you suspect you may be doing that in your relationship with him, it could bring tremendous gains. Whenever we talk with a T about stuff outside our relationship with them, it's second hand. Valuable, but still at a distance and the challenge becomes for the T to bridge that distance. When we can bring the past into the present relationship in the room, it's immediately accessible for both of us. A lot of healing can come from that!