What can I do to reshape my buttocks?

It is very difficult to determine the exact size and shape implant you will require to best match your ideal butt image without an examination by a board certified plastic surgeon. Not just any board certified plastic surgeon, but one with many years of frequently performing breast augmentation surgery including different approaches, techniques and implant choices. This is because several measurements not to mention your breast characteristics are needed to determine the optimal implant size to obtain your goals. Without knowing these dimensions it would be difficult to make this determination. For example, the existing base width of your buttock will determine, in many cases, the maximal volume per implant profile that you can accommodate. Liposuction of your lower back and fat transfer is also helpful as an adjunct to implants. Using “want to be” photos however are useful if simply provided to the surgeon.

Allow me to answer your question as best i can without any photos. Also, please keep in mind that an in-person consultation with physical exam is first necessary before any legitimate evaluation or final surgical recommendations are made. If your self-description is accurate, you may benefit from both buttock augmentation (implants preferred for a permanent result) and some skin removal procedure(s) such as a lower and/or upper buttock tuck. 
There are currently only two acceptable surgical options for augmenting and enhancing the buttock/hip area: 1) Brazilian Butt Lift (aka BBL) - utilizing the patients own fat from liposuction then transferring it into the buttock and/or hips and 2) Buttock and Hip Implants (soft silicone rubber implants that cannot rupture &/or leak). Both are options but what it comes down to, like any surgery, is proper patient selection and long-term results. 
Unfortunately, because ~80+ % of the fat transferred will melt away within a year, most patients are not good candidates because they lack an adequate amount of fat to harvest. Beware, if a surgeon asks you to purposely gain weight (i.e. fat) before BBL, know that the fat you lose first as you get back to your baseline weight after surgery is that very same fat that was transferred into your buttock, hence your new buttock shrinks first, fast, and the most...so don't fall victim to this recommendation. Even those patients that had adequate amounts of fat pre-operatively, still end up seeking buttock implants after a year or so because most of the fat transferred melted away leaving them with minimal result. 
 Although overall using your own fat is relatively safe, it not infrequently melts away unevenly leaving one butt cheek bigger than the other or with dimpling or hard fat cysts.  The one serious complication that can rarely (< 1%) happen is "fat embolism" in which some of the fat gets into the blood stream and travels up into the lungs, heart, and/or brain causing serious problems including death (~0.05%). This complication is even more likely to happen with the larger amount of fat being transferred. This is even more likely to happen when using fillers like liquid silicone, PMMA, Sculptra, and hyaluronic acids. Also fillers, when injected in large quantities, have a relatively high infection rate, guaranteed tendency to migrate away from the original area they were injected, and almost always stimulate a lot of inflammation with a subsequent and disastrous amount of scar tissue/hardening.
Thus buttock/hip implants are a relatively very good, safe, and permanent reliable option for most patients seeking buttock/hip augmentation. 

Due to the skin excess as well as volume atrophy, you may be best served with a buttock lift and fat grafting for volume.  You have to be fine with the scars and this has to be discussed with your surgeon.