What's wrong with my chin/neck? I don't have a jaw line and my chin goes down. What can be done to fix this? (photo)

from the limited views presented it appears a  liposuction of this area will give you what you want. If the muscle is involved it would need to be modified as well. See a very experienced surgeon for this. 

Although its difficult to tell from a photo, you might need muscle modification to the platysma to get a change in shape of your neck, in addition to the traditional liposuction and possible chin implant suggestions.  Its best to get a  personal consultation to determine what you need.