Im getting very fat...?

My weight has gotten pretty bad. I now weigh 268 pounds and im 16. Im very fat and i know i have to diet but i just can't do it. I keep eating junkfood even though i know i can't. I can easily finish a full size carton of ice cream in one sitting. My favorite thing to eat is chocolate. I eat several candy bars a day. Because of my enourmous size im not active since i can't really run or anything. So then i just gain more weight by sitting and eating. It's terrible to find out when im putting on clothes in the morning i need a bigger size. The doctor said that if i don't change my eating habits, in 3 months my weight could be about 315. I need help because im getting dangerously fat, and it looks like i'll just keep getting bigger.