Well, Emoboi, it's good to know that you realize that it's your perception that you believe yourself to be fat, but know that you actually aren't.
It can be very difficult to change a negative thought pattern about yourself, especially when you're so wrapped up in it. Self-esteem can be changed, though.
I didn't love myself before, and when I say love myself I don't mean I have a big head, in fact I find that those kind of people usually are more likely to have a self-esteem problem themselves. But basically what I did, is I got a journal and I'd write down all the things that I like about myself and I would ask what others liked about me if I got stuck. And I keep adding onto that list every day and the numbers went from 50 to 100, to now 221. And because I'm acknowledging good things about myself and realizing that I'm not a boring person and that I was more interesting than I thought I was.
But for example, I wanted to be taller at one point. I didn't like my height, but I said to myself, "I can't change this, so why not be proud of myself the way I am?" and I started to change my way of thinking. I would list all the good things about being shorter and I really got pumped about being the height that I am now.
The hardest thing is making yourself believe it, because you don't want to just say, "Ok, I'm going to believe I'm not fat... but I don't really feel that way." You kind of have to be attuned to what your thoughts are.