Hey there
I remember being your age and having the same feelings!
Just remember, if you start good eating habits NOW, you will have them for life! Yes it seems that all the other girls can eat what they want, but when they are 18 + they will be having a hard time with keeping their weight down AND THEN they will be more likely to head for an eating disorder (Not a healhty thing to do). Where YOU on the other hand will KNOW how to eat healthy, WHAT to eat to keep your weight in check, and YOU WONT NEED TO LOSE THE WEIGHT. You will have already set in stone good eating habits.
Also, get yourself involved in some sort of sport. Anything really. Just try a few things and see what you like. You may not be good at first but once you keep doing it, your talent will come through.
To answer you question about how I stay motivated, well I just keep in my head that I am healthy and I want to stay that way (I have had cancer twice)...Also the benefit of eating healthy, and exercising, is the weightloss,
So deary, it sounds like you have a great Mom who will support you. Use her! Even ask her to be you exercise "buddy" I am sure she would be thrilled (and honoured that you asked) to spend time with you in that way!
Keep you chin up and remember, in 10 years, all those girls eating fries and bugers will be asking YOU for help!!!! just focus on YOU andnot what "they" get to eat.
P.S- if anyone comments on you "not eating the junk" just tell them that you like the healthy stuff more.
Iknow its hard, being 12 to go against what everyone esle is doing, but believe me you will like yourself better if you do!
go get'em!!!