I understand what you mean. To everyone around you you will still be you just slimmer, healthier and fitter. You can still have a joke with everyone, be center of attention if that's what your used to. But the problem comes in your own acceptance of who you are.
I lost six and a half stone once, mentally couldn't cope with it, it didn't matter how many people praised me for my achievements, I didn't feel I was me any more. I felt healthier and fitter but when I saw myself in a full length mirror it wasn't me looking back at me but a complete stranger. I couldn't cope with it so put all the weight back on again and more. Then I felt normal, the true me, others may find that hard to understand but I have been over weight all my life so large was normal.
Since being diagnosed I've been low carbing and loosing weight again, I know I have to do it and I'm determined to continue but it will be hard seeing the new me again when I get there. the difference is I now know I have no choice I have to accept what will become the new me but this time I will be brave enough to ask for help in my acceptance when I get there.
Don't write yourself off, accept that you are still you and embrace that you are a new you with new experiences in front of you. Accept all changes for the good and remember each experience we encounter in life changes us into who we are today. This will just be another change, a monumental one understandably, but just another change to accept and move forward with. Keep telling yourself your still you, it just takes a while to sink in.
Good luck and take care
Maggie