Willow just wanted to tell you that you are not alone.
My son is 3 weeks old and i have had similar feelings that you do, my DH just went back to work on Monday and i had no idea how i would get through a whole day on my own.
coming home from hosptial was really overwhelming for me, thinking this is my life now, totally turned upside down again.
ok so its only day 3 of dh being at work but its probably not as bad as what you are anticipating.
im sure some days will be tougher then others, i remember with my first son everyone telling me the first 12 weeks are the hardest and it was very true.
I havent really given you any good advise but i think its great you are talking about it because it all helps, your hormones are still all over the place too which would be a big contributing factor to how you are feeling.
is your DD being a lot of extra work atm? My DS was driving me nuts the first 2 weeks, i think he just wants extra attention, they do what they can to get it (if it means being naughty, pulling everything out, getting into mischaief etc) he has this week started to get back to his normal self, so i hope if your DD is a bit unsettled she will get used to having her brother around soon.

Hope your feeling ok