What is this pocket of fat on my chin/under my lip? How to I get rid of it?

Thank you for sharing your question and photographs. Unfortunately without an in-person examination it is difficult to offer advice. I would seek out your PCP to work up the mass which may include a consultation with a plastic surgeon and/or oral surgeon.

this is normal for your anatomy and likely is just the muscle. this can be improved with botox and some fillers to the area but nothing will make it go away completely. start emeragecosmetics AERIFY x ANTEAGE dermarolling to improve the texture (see link). Best, dr. Emer.

Dear ErinW02,
it is hard to tell for sure without an examination. If you are considering a surgery, I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery