"Ghostly essence" refers to the feeling that there may be a spirit or ghost present, or a strong memory that feels like it's somehow happening in the present. It is kind of a feeling more than something tangible. In this case, if there was a "ghostly essence of you", it may mean that it feels as if "you" should be sitting in the chair or are sitting in the chair. It feels as if "your" spirit is connected to the chair.