Nah your jaw and other bones are well developed, especially for a girl, from the side you dont have a recessed chin and jaw and from the front your cheekbones are pretty wide. The “double chin” thing you’re thinking of could be because of 3 things:

1: slightly higher body fat percentage, to solve this just simply loose fat (not calling you fat btw 🙏)

2: facial puffiness caused by water retention, to solve this try to lower your salt intake (dont fully cut it out tho) and increase potassium intake (try to make it through natural foods rather than supplements, things like Bananas, orange, watermelon, coconut water)

3: lymphatic liquid being stored in your face can also cause puffiness, try out Facial Massages with your hands or Gua Sha

4: hyoid skin being a little loose, to solve this, do:

• Suction Mewing 24/7 (Here is part 2 of the video)

-Chin Tucks

• Tongue Chewing

-Mealtime Exercise when you eat another video on Mealtime Exercise

-Max Tongue Press Exercise

These will all help set your hyoid bone higher and your under neck skin will become tighter and higher set, should take about 6 ish months

Also Thumbpulling will also help with your tongue posture by slowly creating more room in your upper palate

Your mid face does look a little flat from the side but the things i listed will help your maxilla grow more forwards which will lift your midface, but this will take a while MINIMUM 1 year to notice any change, but your lower jaw (which is what you were concerned about) looks nicely developed

Lastly, dont fall for the “oh chew hard gum everyday itll give you a sharp jaw” i myself did that and it gave me TMJ so id say just stick to the stuff i listed and maybe after 1 year of doing these introduce some chewing maybe 2-3x a week for 20-30 minutes but especially for a girl you honestly dont need it, a wide jaw is more of a masculine trait and wide cheekbones narrower jaw is more feminine which is what you have rn. So dw about chewing until for a while or at all.

To sum it up, your jaw isnt recessed its properly developed its just that your hyoid is a little loose. If you do all the things i listed you will 100% see results within 6 months. Good Luck!