Need motivation fo weight loss!?

I am a 14 year old girl, and i weigh 79.1 kg. wish is over the weight i should have. Thing is, i have tried so many different diets like: Dukan, Weight Watchers, Atkins, 3 day plan and many others. And i do lose weight, for the first couple of days but then, for some reason i give up. I eat chocolate or i buy things from the canteen at school, and i gain the little i had lost. Please, does anyone have any ideas to what i can do? maybe you lost weight with a diet that's great. or you found a really motivational video but whatever it is, i really want to know. thank-you :)