Hello dear!!Thanks for the question and provided information as well.Looks like you could benefit from a fat transfer procedure, but unfortunately, without pictures, it is very difficult to give you a proper answer. l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.Good luck :)
Asymmetry that you describe may be due to asymmetry in your spine and or pelvic bones as well as soft tissue. Often it can be camouflaged with liposuction on one side and fat grafti on the other. It is impossible to speak specifically to your problem without at least pictures but certainly an in person exam is needed. Please see a board certified plastic surgeon with specific interest in buttock aesthetics. I hope this helps.