Why do i get into a bad mood when i diet and get off at people?

My mom says don't eat sugary/junk food and i could lose 15 pounds because i am currently 15 pounds over weight. Being 14 years old girl and having cravings i get in a bad mood when my mom says eat vegetables instead of ice cream and Doritos. What should I do? When i am not in a diet I'm a happy sweet girl. ... This is the 3rd day that i'm dieting with ( no sugar/junk food/bread )