My therapist is very interested in dissociation and uses the term openly, but that may not cover all of what your therapist is saying. Mine also says that we lose parts of ourselves, parts that get stuck with the trauma. That makes sense to me. It also is really clear that it must be true since DID is caused by repeated trauma and is the extreme instance of parts of ourselves being lost from the main self.

Some people who write about this believe that all of us have mildly dissociated selves. That we are a collection of different selves, and some of these are not as acknowledged or accepted as others, sometimes because of trauma. These parts are felt as "not me" and when they come into awareness, it really turns things upside down. Still that is the first step toward reintegrating. So I guess what's important is to see the loss or whatever as not a permanent type of thing or as a defect.