When you go on a diet. how do you stop your self from eating junk food?

im trying to loose 3stone and i cant help myself! i have chocolate and sausage rolls 4 breakfast... i have junk all day! i have a take away almost every night. people tell me im not fat, but i used to be 8stone and iv recently had two children, and now im 11stone. i want to be 8stone again so i keep saying im going to try my best and i do for a few horus, then i cant help it any more.... how do get past it?