Why do people believe that being transgender is a mental illness? Can mental illness affect small children, be lifelong, and have no treatment?

This seems like so much more than a mental illness. It seems like who I am. Gender determines almost everything about you. Everything about being a boy felt wrong and everything about being a girl looked right. When I was 7 and my cousin and her friend dressed me up as a girl I was totally humiliated. I expected to look in the mirror and be so embarrassed. Instead I looked in the mirror and felt like I had just looked in the mirror for the first time in my life.

Why do people believe that being transgender is a mental illness? Can mental illness affect small children, be lifelong, and have no treatment?
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