Why cant I stop eating junk food?

My weight and appitite has always fluctuated and so has my eating habbits. I have never been totally happy with my weight I seem to go through phases of eating healthy and then pigging out on junk food it,s like I,m a devil possesed when I start piggin out I just cant stop. And then I feel really bad after ive done it. Its really startin to get me down now why cant I pull my self out of theese stupid eating habbits I dont want to eat crappy junk food but the more I tell my self I,m not gunna eat it the more I do. If have a bowl of bran flakes and an apple for my breakfast 15 mins later I end up cooking 2 bacon and egg ies afterwards along with 2 packets of crisps has any one got any ideas of how I can disciplin my self because its driving me mad!!!