Why can't I stop eating once I start?

I know people generally tend to binge on things that are high in fat and refined sugar, but I binge on healthy stuff. I'll make a healthy, balanced lunch but then I'll think to myself 'I fancy a little something more' and then it just goes down hill from there. I eat loads of low fat cereal like bran flakes straight from the box. Low fat cereal bars, fat free yoghurts, toast, crumpets, pitta bread, salad, malt loaf, fruit... I'll eat and eat until I feel so full it's uncomfortable. I never seem to feel hungry and I only feel full when I am REALLY full (i.e. After a binge) Why is this? How can I get my hunger and full signals back? (I'm not gaining any weight (I'm currently a healthy weight for my height) from doing this, I probably do it 2 times a week. I don't purge either)