Can I answer this question? I use to weigh 140lbs (back when I was about 17 that is) so I was thin once....does that mean I can't answer your question?....ah well, I'll answer anyway.
I think when you have been large all your life (or in my case almost half of my life) it's very difficult to ever imagine being anything but larger. It brings up all kinds of quandries, like, will you recognise yourself when you get to a thinner weight? Will other people recognise you, will they think you are better looking, or prefer your face when you were bigger? If you have never given them anything to compair to, then it must be werid seeing this person emerge who you have never seen before.
My DH met me when I was 30lbs or so heavier than I am now, I keep telling him, he is married to a different woman. And kind of he is.
In answer to your original question, I have serious trouble seeing myself as thinner now. I still go to walk through cars and think, will I get through there? I still worry about getting on a plane and not being able to get the seat belt round me, I still worry about not fitting into toilet cubicles etc etc I know that since I have lost this weight I will be able to do those things, but when you've lived with being heavy for a large amount of time, it's very difficult to get out of that mindset.
Now, for you, I assume you mean you have never been what anyone would call a 'normal' weight, so in a way, you are going to take yourself so far outside of what you know, that yes, I believe you will have a problem (problem is the wrong word....difficulty) getting your brain to accept that you CAN be a normal weight.
I met someone once who was 450lbs, she had already lost 50lbs and was losing weight steadily. She told me she had set herself a goal to make it to 300lbs and then she would stop. When I asked her (shocked in fact) why she would stop she said because she couldn't see herself going through the dissapointment of not getting lower than that, so 300lbs was her target. Looking back I realize now that she was setting herself a goal....it was a short term goal, and one which she felt she could get to. I didn't understand that at the time, but when I started LC I finally got it.
I met her again 3 years from that and she weighed in at 180lbs. I said, I thought you were going to stop at 300lbs! She said, I got to 300lbs and stayed there for a few months, I felt like I had to live at that weight to actually prepare myself to move on.....but I did it, I set another target to weigh 200lbs, and I stopped again, this time it freaked me out, the person in the mirror wasn't me, so I again got use to myself at that weight, and then I took the final weight off.
I asked her if she still 'felt' large....she said she did. The woman looked like an average weight for her height, if you saw her in the street you would not look twice....she had undergone a complete body lift and she looked fantastic, but in her mind she still FELT 450lbs.
(sorry long post)
I then met her a year AFTER that and she looked fantastic, she told me that it had taken so long for her head to catch up to her new lower weight....and she was STILL working on it. She said she looked in the mirror every day of her life and still barely recognised herself!
So, what I am trying to say is that you might never think you can get there, but if you take things slow and actually pace things - maybe even breaking it down into sections like she did, you can get there.
I KNOW now that I will be thinner...I might never see myself as being stick thin like a model, I don't think that's ever going to happen, but I do know that I will achieve getting to a 'normal' weight. And even if it takes me the next 2 or 3 years to get there, get there I will. And so will you.
But yes, I do think it's very difficult to get out of the shell you are in right now and SEE the person you could become. It's like asking yourself to look at someone you have never met before. But you'll get there