You have to set reasonable expectations. You can't do 1,000 burpees in 10 minutes and you will never change the biology of your body. If you're bottom heavy, you will likely always trend that way when gaining weight. If all the women in your family get saggy underarms by the time they are 40, you probably will too. However, you can tone and shape that bottom, heavy or not. You can strengthen and tighten your triceps to help that underarm sag. But remember: there is no amount of exercise that will completely transform you into someone else — and you shouldn't want it to. You were made to be you, unique and special. You can be the best you possible, and that should be your goal. Once you accept that, you can stop fighting your body and start working with it to achieve your goals.
