I'm sorry that you are so upset. I think pg hormones alway make things harder to handle as well.
I must admit to be a little bit excited for you, i am certain that if this is a boy, you will be consumed with love for him, just like with your girls and you will experience the joy of both genders.
I really feel that it will be a good thing for your girls to have a brother so that boys are more familiar to them, it may even save them from experiencing GD in the future if they are mums of boys.
In my experience GD gets better over time and i wouldn't be at all surprised if it disappears entirely for you once you meet your little boy (if it is a boy).