Hello & welcome,
Prognathous mandible or mandible protruded more than maxilla
( called as Class III Mal-occlusion ) usually leads to lips drooping down or look bigger.
The best advise here is to approach an Orthodontist & undergo clinical examination.
You may require to take cephalogram or Ortho pantomogram ( X-ray or Radiograph orthodontist uses for diagnosis ).
Treatment exactly depends upon your age. If your age is less than 18 yrs that means you are within growth period. So, it will be very easy to treat by placing chin caps along with other required orthodontic appliance.
But if you have crossed your 18, then either braces or orthognathic surgery
would be required to get desired results.
Don't use to pull your lip back or in as this will put pressure on lower anterior teeth causing them to retrocline which will again add to situation.
Even when orthodontist feels that only orthodontic treatment will not suffice, then lip surgery
can be done to get maximum best results.