I tend to have the same problem. I saw or read somewhere that there's this eating disorder where people literally can't feel "full", like there's this mechinism in the brain that is supposed to flip that switch doesn't work right. I've always thought that I had the same kind of issue, but with reassurance. I can drive people crazy with it sometimes. I've gotten a little better about keeping it to myself, but it's still there. I know how you feel.
I think that it might be an anxiety thing, but I'm not sure?