You are not alone. I still have the image in my head of me when I was 17 and a size 12. I think that's the smallest weight I remember being at (not counting as a child) and I *think* I was around 200 pounds. So when I visualize an outfit, Its on that body. When I think of an event, That body is there. It didn't occur to me I was a different size until shopping for a bridesmaid dress for my sisters wedding- when I cried in the dressing room because the store only had sample size 7s and nothing fit and it felt horrible.
I've realized there are 3 types of people on Spark:
Type 1: You see yourself as a skinny person
Type 2: You see yourself as you actually are (rarest of the 3)
Type 3: You see yourself carrying an extra 100 plus pounds of fat on your body.
You and I seem to be type 1. While I think this is a good thing, we need to accept where our body's actually are and uses these mental images of ourselves as goals and guides as apposed to where we are.
Don't worry, you're okay. I think this is normal
I think the reason why you're having trouble sticking to losing weight might be a comfort zone thing. It took a lot for me to be able to work out in public simply because I was afraid of people watching me screw up moves and it took a lot or me to make myself realize No ones watching me.
best of luck honey.