Answer:

Failing to feel full can sometimes signal the need to keep eating even though the body's fullness mechanism (called satiety) is doing its best to signal eating to stop. Sometimes it reflects a psychological need to eat (food can have a numbing effect and can help a person escape from feelings they don't like). Seeing a therapist to help understand the role food is playing can be helpful for some people. For others, they just get used to eating a lot and it becomes habit. There may a physical basis as well. Sometimes medications make people eat more, as an example. You should probably ask your doctor just to be sure everything is OK on the physical front.

Your body's defense against overeating is probably in there somewhere, but it is being overwhelmed by something stronger -- your desire to keep eating. A good goal is to get the eating under control and start paying attention to the stop signal. Eating healthy foods is a good start. Slowing eating can also be helpful. Having something to do (from a list of activities) when the urge to eat gets really strong can be a good aid.

There is also a very good book you might consider. It is called Overcoming Binge Eating (by Dr. Christopher Fairburn) and is a step-by-step guide for the person with compulsive eating problems.