I hear what you are saying! Ms Mandalinn has good info on why this happens.
I know that for me, to overcome it, yoga has helped. And not because it is yoga! It is because I have been FORCED to LOOK IN THE MIRROR every single day while in form fitting clothes.
You might try taking a stretching class or go to the gym every day and take a class where you have to look in the mirror every single day. I think this is super important, because if you look at yourself, and then at others around you, you'll get a sense of your current size.
You could do this at home, too, just by getting into FORM FITTING workout clothes and just taking a look every day to reprogram your brain from it's old image of you to your new shape.
OR, you could take photos of yourself today. And place them side by side with old photos when you were larger. What I WOULD do is just cover up your face, because without the head on, you don't "see" yourself, in that you'll make a better unbiased comparison, instead of having your brain ID the "old you"...this will help you see the differences, and pretty soon, your brain will be re-programmed!
Kira