Been there, done that :)
I really wanted a girl first time round, and when they showed me the boy parts at the u/s, I was on the verge of tears. I managed to hold it together, but I did have a good cry when I got home that day. I was sad for a couple of days, but then things started to get better, and by the time he was born, it was all put behind me. I still wished for a girl, but I didn't regret having my boy one bit.
This time, I was really nervous leading up to the u/s. Since it is my 2nd child (and perhaps my last) and I had swayed, I was really hoping that it would be a girl. I had pretty much prepared my self for a boy up till then, because I didn't want to get my hopes up and increase the disappointment if it turned out to be a boy again. I also knew, though, that whatever it was, I would love it no matter wha, even though I would be sad not to be having a girl. when the tech said it was a girl, I almost couldn't believe it. Still can't sometimes - I keep thinking that when the delivery day comes, the baby will come out and they'll say "it's a boy" I don't think I'll totally believe it until I actually see her for myself in the flesh.
So many of us have been there. I totally understand where you are coming from. Just try to remember that if it does turn out to be a boy, it's ok to grieve for the girl you aren't having. it doesn't mean you'll love your little guy any less. you'll get all the support you need here.