Chin connects directly to neck - how can I fix this? (photos)

Your chin in particular is vertically short which is why you have as double chin (technically a pseudo double chin) since the cause if not excessive fat. Vertical chin lengthening (with some increased horizontal projection as well) will solve the double chin issue and make the lower face in better proportion to the upper two thirds of your face. To achieve vertical chin lengthening the options are a custom chin implant or an opening wedge bony genioplasty.

your lower jaw is underdeveloped and the distance under your chin to the throat area is 'shorter' so the 'fullness' can be accentuated. As well the 'height' of the chin/front of jaw is short emphasizing the same issue. One can either have orthodontics and jaw advancement surgery or a genioplasty which masks the issue