You look GREAT!
Be patient with your skin, you are very young it will fall back in place, keep tonning up, don't be so hard on yourself about the saggy skin.
If you like you can rub some olive oil on your skin or even cocoa butter, it won't make it snap back in a day but it is really healthy for your skin and it helps bring back some elasticity and it will help the process out by helping any strech marks almost disappear.
All you can do (without surgery) is keep your skin moisterized and also, I know this may seem weird but in all honesty I've heard for loose skin try to wear some kind of a gurdle when you go to bed, or even like some cloth, tie it around the problem areas tightly but still be able to breathe of course, and this should slowly help too.
A lot of women do this after pregnancy to help the skin attach back to the muscle or fat.
HOpe I helped some.