Was there any event that triggered your body to "catch up with your mind"?
What have you been told about your body image by important people in your life? Do you remember?
My mum recently decided to start telling me I had no idea what I looked like and that I'm much fatter than I think I am.
Whereas my dad recently told me that I looked great, I was just hiding it because I had no self-esteem.
My reality is somewhere in between.
I believe you that you are thinking positive thoughts, but apparently there is this voice inside you that is not convinced by such positive thinking. And from what you are writing I don't think it's gonna magically disappear when you reach your goal. I can't really tell you how to get rid of that voice, but I'd advise you to watch it very carefully, because we all know where it can lead if you are slim and your head still does not acknowledge it.
Good luck on your journey!