Aw, don't feel so bad about yourself. You're not a bad person to feel this way!
When I was pg for the second time, we already had a ds, and, while dh harboured a secret desire to have a dd, the truth was that neither of us minded what we had. Then, at the 20w scan we were both independantly convinced that we had seen clear evidence of the baby being a boy. I mourned for the fact that I wouldn't get to do all the girlie things with my daughter, help guide her into womanhood and worry about her boyfriends, not to mention cry at her wedding and stay up all night worrying throughout her labours (see, I'd already constructed a whole lifetime with my 'daughter'). And I felt an idiot feeling this way, not to mention ungreatful for the priviledge of carrying a healthy baby. Because dh didn't want to know what we were having, we never talked about this until after the birth, so I felt very alone in my thoughts. I grieved. But gradually, by the time I gave birth, I had come to terms with the idea of having two boys, and could only think how fantastic this would be, and what a voyage of discovery (I too have both brother and sister).
Then, of course, I gave birth to a girl. What a shock! I didn't believe it until I saw for myself. And, like a post-natal, hormonal, nutter - I grieved once again! This time for the boy I didn't get! It took me 7-8 weeks to even admit this to myself, and probably another month or so to come to terms with the fact that I was/had been disappointed not to have a boy. Of course, another spell of guilt and self-bashing. I couldn't focus on the delight of having the chance that my dreams about being the mother of a daughter could come true. But the truth is that I'm not disappointed to have a girl. Like I came to terms with thinking that I was going to have a boy, I came to terms with the fact that I had a girl and love her to bits, and feel priviledged to have her. Actually, I fell in love with her the moment they lifted her off my chest to check her over.
It all comes around in the end. Right now you feel all mixed up because you have to suddenly readjust your dreams - while in the throes of pregnancy hormones, which don't help at all.
You're not nasty, foolish, evil or even ungreatful for feeling this way. Love yourself and your baby - you will in any case, however long it takes for everything to fall into place.