I think a lot of women go through this. Have you talked to your doctor about it? I'm not sure if it's depression or just anxiety. Talk to your husband. I know you're scared but maybe just talking to him about it will help allieviate some of the negative feelings. It's perfectly normal to have mixed feelings about having a baby. You are taking on the biggest committment of your life and I would be worried if you didn't have any concerns. It is a huge deal to have a baby knowing that you are responsible for raising that person into responsible adult.
But, you know that this is going to happen and you need to be as prepared as you can be (and even you said yourself that you can't fully be prepared for motherhood). Start getting things you know you'll need for the baby. Maybe doing some shopping will help get you a little more excited.
Talk to your husband, I think it's really the best thing you could do for yourself. You might find out that he's scared, too, even if he is excited. Also, be prepared that after you have the baby you might not just fall into your idea of the perfect mother, but that in time, everything will just sort of click. The best thing someone told me was that I might not have that bonding moment with my baby immediately (and I didn't) but when he was about 3-weeks old, I did and it was amazing. Things might not start out the way you expect or plan, but I have no doubt that you will eventually love your baby unconditionally, you'll love being his mother and that you'll wonder, "why was I so nervous?" No one can prepare you for being a mother, but you may just find it's the best thing that you ever did.
Good luck and best wishes!