I'd suggest writing about your feelings in a journal or something (and probably either locking it away somewhere or destroying it, because it would NOT be fun explaining that to any member of your family that might come across it!)
I felt just about the same way as you do on the day I found out I was having a boy. Even though it's my first child, and I'm 20 so I have plenty of years ahead to try for a girl, I NEVER wanted a boy. I didn't really want kids at all, but when I found out I was pregnant, I kept having dreams about having a daughter now and one later, and I really liked those dreams, so the ultrasound was a killer. I seriously considered adoption (I'd already ruled out abortion before, because while I'm pro-choice all the way, it just wasn't the right choice for ME), and sometimes still feel like I should do that, except that the whole family knows about the pregnancy and I know my boyfriend's family would judge me...plus it would be difficult for everyone since it's the first grandchild. I also desperately wanted to stop getting prenatal care, stop my vitamins, not get screened, everything (for the first day or so).
I allowed myself to feel as low as I was feeling, because if you don't acknowledge it, it can get a billion times worse. The whole time, I told myself "if this awful, sick feeling doesn't go away in a week or two, I'm going to go see a therapist." I found out last Monday that it was a boy, and from reading and posting here, and talking to some other understanding people, I've been able to, for the most part, will away the negative feelings to the baby. I still feel some disappointment, but I'm also finding some peace (which is totally unlike me, to come to terms with such a major disappointment in the span of 7 days!). I can't tell you you'll recover like I did, or as quickly, but I can tell you... it's possible. If you can believe in that, that it's possible that some of the hurt and negative feelings will pass, that can go a long way in helping.
Also, I'm guessing that the father of your children (if all share a father) is at least somewhat of a positive part of your life... if you can focus on him as your image of what a man is, rather than all the disappointments you've had before from men, maybe that can help. Or just try to imagine the opposite of all of these disappointing men, and believe that you and your daughters can help your son to become as close as humanly possible to that, and maybe the future with a boy won't seem so daunting and unpleasant.
I'd say-- let yourself feel how you feel, while believing it's possible to recover. That one simple belief can do a world of good.
The best of luck to you.