I seem to suffer from the exact same thing. I can leave the house thinking I look fabulous, and then I see the picture and wonder what the heck I was thinking.
I deluded myself so badly about my size, and the worst was when I lost the first 100. Who loses 100 pounds and doesn't look waaayyyyy better? Someone that still has another 300 to lose, that's who.
The first step is making the change. And it's tough when your illusions about yourself are shattered. But you'll feel so much better when you get there.