What will fix the sagging of my butt toward the middle of my thigh a year after a BBL? (photo)

I would say you need liposuction of the inferior and medial buttock to blend in a nice  round shape.

Hello, thank you for your question. It is hard to tell without a physical examination. You may benefit from a liposuction, I recommend you to schedule a consultation with a board certified plastic surgeon.

Hello, thank you for the question and great photo.. after analysing your case.. I can say you're a good candidate for a BBL revision, since the job the last dr. did to you wasnt that effective. A revision woudl be good for you, this would fix your problem, give you great results and improve the look of your butt. 
Best of lucks.
Dr. Luis Mejia

Dear ccoulter0006,
I understand your concern. However, without a proper assessment it would be difficult to determine what is wrong. It is best that you visit your plastic surgeon for further assessment. Only after thorough examination you can get proper recommendations and advice.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery