I sometimes get this too, I see the photo's that I've put on here, I am wearing smaller clothes, I *know* that I am a lot smaller than I was, and yet when I look in the mirror, I struggle to see what I know to be true.
I think the mind really does need time to adjust to how different your body becomes, and because we se ourselves all day every day, it is harder for us to notice changes (both good and bad) quickly.
If you could stand next to your old self, and actually see for real how you used to look, then it'd be so so apparant how different you are now.
Just remember, you are wearing a size 14, you ARE a size 14, and you HAVE to be slimmer to have dropped 4 dress sizes (and what an achivement!)
Keep going, stay strong, you'll see youself as you really are soon enough