Big hugs! Maybe it's just the shock of it all that has you so worried about things. Have you thought about going to counselling about it, just to try to sort out your feeling and put an alternative plan in place. You might be surprised to realise though things will be different, they will be different in a good way!
I do believe that it's all about how you percieve things. Yes, babies are hard work, and two children is hard work, but you can still do other things besides being a mum! I have worked and/or studied while my children are very, as has my DH, and I stress alot too. My kids are terrible sleepers too, my DD is 3 3/4 and has just starting sleeping through int he last 6mths, DS at 2 1/2 still doesn't sleep through the night. It's tough, but if you want those things in life, having another bub shouldn't stop you. I tmight make the time frame a little longer, the journey will be different, but you can still get there!