Hi Chica, I really feel fo you at the moment. My first was unplanned , and I went through almost the same thought process. The first thing I remember doing when I told my partner was getting out the calculator which is sad.
In the end I just thought , ok so it's happened , it's not when I wanted, but lets go with it for a bit. As the pregnancy rolled on, I really did start feeling more positive and excited so I am sure things will settle down for you soon, especially when you get out of the nasty first trimester.
If it helps I had number 2 and 3 kids within 13 mths of each other and number 1 was only approching age of 3 , and as it turns out , it was probably the best thing, what you lack in time with 2 , it is made up in their brotherly/sisterly bond and playmate to grow up with - great. And when the baby is small, you actually overcompensate and the older one gets loads of attention, time and love.
Wishing you well - JJ