I had a lumpectomy last year. I was originally advised to have a mastectomy because I had two cancers in the same breast. Like you, I couldn't process it all, changed surgeon and had a lumpectomy instead. My operated breast is now much smaller and harder and still discoloured after a year. I still haven't worked out how to "pad" my operated breast to look the same as the other one, when I'm wearing something quite tight-fitting. I've only just reconciled myself to looking at myself naked in a mirror. I know it's not quite a mastectomy, but I wanted you to know that I can empathise with how you're feeling. I've persuade myself that the rest of me still doesn't look too bad and, at least, I'm still alive and hoping for a good few years yet.
Incidentally, I hate to be the bearer of bad news, but it's possible you won't be offered a reconstruction. I was assessed for one, but in the end, it was refused because I'm diabetic and it was reckoned that the risk of infection and/or rejection was too high. It freaked me out at the time.
After a year, I've learnt to live with it. My advice would be to use any support which is offered. Is there a "Maggies" at your hospital? There are other women who are going through exactly what you are. My breast cancer nurses were wonderful. They were used to shouting, screaming and crying and I was lucky that I could always contact them.
Whatever happens, I wish you well. You wouldn't be human if you weren't upset and worried. Let us know how it goes. 