|This is fat. This is not you. Learn to recognize the difference.|
I am happy.
I am a woman.
I am American.
I am a triathlete.
All of these things, for me, are true. They are one word descriptions of the state of being. They are one word descriptions of what my being (me) actually is. By saying that you are fat, it implies that it is part of your current mood, ambitions, your genetic makeup, or your gender, your nationality.
Is "I'm fat." really who you are?
That would be like saying, "I'm apple." or "I'm goat." It makes no sense at all. You can't actually be an apple or a goat. Of course you could say, "I'm an apple." or "I'm a goat." But you still can't say, "I'm a fat." That just sounds wrong. Furthermore you are not an apple or a goat - those things are not you. They are a fruit and an animal, respectively.
Fat is a noun, and should not be used as a descriptive word. It seems the proper phrase is, "I have fat." Maybe it's "I have a lot of fat." or "I should lose some fat." But, it's not who you are as a person. Also, stop saying, "I feel fat." That makes it sound like you are sitting there, touching a mass of fat. That's just gross.
So, stop calling people, especially yourself, fat. It doesn't make any sense. Just be PHAT instead (Pretty Hot And Tempting).