What can I do to fix my saggy butt? Lost a lot of weight. (Photos)

 First, congratulations on the weight loss. However unfortunately once the skin is over-stretched it is unable to recoil back, even with healthy nutrition and exercise. Usually what I recommend in these cases is to "reinflate" the buttock first by placing a buttock implant restoration of volume and fullness. Then  6+ months later remove whatever residual skin remains with a "lower buttock tuck". Glad to be of help.

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.

Hello dear!
Thanks for the question and provided information as well.
Based on your pictures, you are a good candidate for liposuction with BBL and buttocks tuck, to remove the extra skin, and I think you'll get very good results. 
Liposuction allows your plastic surgeon to transform your entire shape. You can improve your abdomen, hips, back and flanks, and of course your buttock, to give you a beautiful heart shape.
In general, you need to be physically healthy and at a stable weight, have realistic expectations, be a non-smoker and your BMI needs to be 30 or less.
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 :)

Congrats on your weight loss! Unfortunately, we are often left with excess skin that cannot be corrected with diet and exercise. I would recommend a buttock lift where the excess buttock skin would be removed by an incision at the buttock crease. This provides a much smoother and tighter buttock but you will be left with a scar at the crease. Please make sure you have realistic expectations about your final result and have an in office consultation to be properly evaluated.

Congratulations on the weight loss! A buttock implant would look wonderful and really help to fill up that deflated skin.  However you may or may not have some residual loose skin.  If so it can be removed a few months after the implant has settled.  Be sure to choose a Board Certified Plastic Surgeon that specializes in buttock implants and has a extensive experience with such procedures. 

The solution to your welcome problem is a combination of fat transfer and creams. Though your texture won't be restored completely, your improvement will be quit noticeable. Finding someone who can do it may be a challenge, but it is also manageable, See a board certified plastic surgeon, who operates at a certified facility and does a ton of BBLs and ask her, or him; "how many fat transfers to the butt did you do last year, and may I talk to someone who had the same problem who you helped?" Don't settle for vague answers, and don't let anyone operate on you who can't find a single patient like you willing to talk. You got here by being motivated and persistent, congratulations...now make sure you are in good hands.

With deflation the tissues sag. You might need a buttock lift or even a buttock augmentation. You would have to be seen in person.

Congratulations on your weight loss! Unfortunately, the excess skin comes with weight loss and it is very difficult to recoil even with diet and exercise. You would benefit from a buttock tuck +/- BBL.  You also need cooling helium plasma to smooth out your skin afterwards. 

No amount of exercise can improve the appearance of redundant or loose skin.  Hence, you may consider buttock lift in which the scar is made within the groin or skin fold for its optimal concealment.After complete healing (about six months), you may consider buttock augmentation via fat transfer (or more commonly known as Brazilian buttock lift, although it doesn't actually lift the butt) to further improve your backside's contour and projection.Your best bet is to consult with board-certified plastic surgeons (ABPS members) who regularly handle massive weight loss patients requiring body lift.  Compared to "average" patients, MWL patients require more extensive approach or additional technical details to achieve optimal results.Good luck.