Gee....does this sound familiar.....! I also have no "real" concept of how I look. As a kid, I always saw myself as huge. Yet if I find a rare photo of myself then (that I haven't found previously and destroyed)....I don't recognize myself. Nah....that can't be me....she's not fat! Then as an adult, I became obese. Self-conforming to my mental image of myself? Now there's a scary thought!
Yet...even at my heaviest, I don't think I really "saw" myself as huge as I was. Which is why I hate mirrors....and hate having my picture taken. I would look at the images....and think HOLY.......! But....even if I didn't see myself as "that" huge....I knew I was much heavier than I wanted to be. So....years and years of yo-yo'ing, up and down, up and down....I found something that seems to work for me. Haven't gotten to where I want to be, but I've gotten closer. And yet.....my image of myself is the same. I can't see the changes! Given the amount of weight I've lost - I should be able to see *something*, damn it! Right? Others can! So yeah, I soooo totally get not being able to see an accurate image in the mirror.
I'm wanting to check out that website you mentioned, just to see how far off I am, in my own thoughts. But....getting back to YOU - here's something I haven't tried yet....I think I heard about it on Dr. Oz's show....LOL I'm tempted to do it...but it requires a second person....and I can't get over the "feeling silly" stage to ask someone to help me. But it's a thought - maybe something for you to try.
Get a BIG sheet of paper.....or several papers taped together. Butchers paper, wrapping paper,....heck even several "poster boards" taped together. Lay it all out on the floor - then - draw an outline of yourself. I'm not talking about drawing an actual line around yourself - not yet. Just draw an outline of how you THINK you are.
And then.....get a friend, and give him/her a different color to draw with. Lay down over YOUR outline, and have him/her draw around you as you lie there.....draw your REAL shape. I seem to remember that "Doc" said...the very act of your laying down, feeling the marker outlining against you, helps you to accept that THAT is real.
Just a thought.

Meanwhile, if you find some other way to really "see" yourself as you are....let me know.