big gender disappointment

I have always wanted my first born to be a girl and when I was told at an 18 week scan I was having a girl I was over the moon. I was told that it was 99.9% certain and the sonographer had 100% success rate, I even asked if there was any chance she could be a boy and was told no. I'd chosen my name for my little girl, I'd bought pink things for her, I'd pictured my life with her.
Then at my next scan with a different sonographer I was told that they'd made a mistake and this was in fact without any doubt, a boy. 
Well now I can't get my head around it. I had told everybody that I was having a little girl. Everyone knew how happy I was and how I'd always wanted this girl and then to be told that actually I never had her at all was so upsetting. I really feel like I've lost her, like she's died, but how can you lose someone who was never really there? 
I imagine I'm going to get comments like "you should be happy your baby is healthy" but that is of no reassurance to me. I feel that in my mind I had already made a bond with my little girl - and after all this time she is a HE. 
At the moment I just can't imagine having a son. I don't know what to call him, I have nothing bought for a boy. I can't believe the sonographer could get it so wrong. It wouldn't have been so bad, but they lead me to believe there was no margin for error. I'm not sad because he is a boy, I'm sad because I thought he was my girl. I don't know how to feel.