I have been in your position and know it sucks! I know how you feel but i can say that 3 yrs later those feelings of total desperation have gone and i honestly feel blessed to be one of the lucky few who will ever know the true joy that can come from having 4 boys, i am sure who will understand what i mean one day. The desire for a dd is still there but the sadness at having lots of boys has gone!
Right now though just try to get through as best you can, it is a greiving process, so let yourself feel the sadness now, it doesn't mean you won't love your son, so try not to feel guitlty. I had my DH tell most people as i would burst into tears if i had to, i had very few negative comments, most people would just say something like "oh wow 4boys!!!' i would just try to smile.
I really am sorry that you are feeling so upset, it will ease and as you know from your others, you will be totally in love with him once he is born.