GAHHH I was like that. At my heaviest I saw myself as only a little chubby and what I saw in the mirror didn't match up with pictures. I always used the old, pics make you look heavier mentality. I was outgoing, hyper, up for anything, always trying to keep busy with some sort of activity.
The first time I became aware of my actual size, the "Fat girl" Mentality set in. I became depressed, didn't try to put any effort into my appearance(where as before I did), and became withdrawn from the public...
When I went through that I honestly wished that I was still ignorant about my size, because I was much happier without knowing how big I had gotten
Now I have issues with trusting mirrors at all because I still see the same person I did 165 pounds ago...but I totally get what your saying because I used to feel like that too
I'm hoping that me comes back when my mind catches up to my body