I can't tell if I am full when I am sitting

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Ever since I started my new lifestyle I can't tell when I am full if I am sitting. However, when i stand up I can tell. It is so odd. No matter how slow I eat I can't tell if I am full while sitting. I could constantly eat for probably an hour while seated and not be able to tell that I am full until I stand up. This makes it really hard to not over eat. Does anyone know why this is happening?