What is wrong with my chin/jawline? (photo)

As I've gotten older I've noticed the lower half of my face is holding so much weight. I already have a square face shape but my jaw seems to just blend into my neck and I have no defined jaw whatsoever. I am only 21 and weigh only 120lbs. There's no reason the lower half of my face should appear so overweight and it's making me quite miserable. I can't figure out what the issue is and what can be done about this. I also have a fairly prominent chin and it's noticeable from the side.

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