P.,
I'm going to respond to this as someone who hasn't struggled with depression in my life. So, I absolutely do not know exactly how you are feeling. BUT, I am a Mom. And I have two children. One is 2 1/2 and the other is almost 6 months.
When I was pregnant with my second son, my first became a Mama's Boy. I don't know if he was stressed about the new baby, could sense my stresses or just a phase he was going through. Regardless, even though he wanted "Mama" all the time, I just couldn't be there all the time. I was put on bed rest for some heavy bleeding and I couldn't lift my toddler. He had to depend on Daddy for some comfort. There wasn't another option. I feel that's the same in your case. I think your daughter needs to be able to be comforted by both parents. And, I would start encouraging this now as it will be harder once the baby is born. I think it would be more drastic for her to all of a sudden "lose" her Mom (in her little eyes) to a new baby. If you ease her into it, she will be used to bed time with Dad, baths by Dad or whatever. Does that make sense?
Ok, the stress of #2...oh boy. I wanted a second baby so much in my heart, but in my head I was hesitant...daycare, diapers, taking time away from my older son, another colicky baby (my first was). In all honesty, we decided to try and after a month, I didn't get pregnant. I figured, "That's for the best. We don't need another." Well, I was pregnant the second month. I was shocked. I think I may have even cried. I KNOW I stayed up at night thinking, "How are we going to do this?!" Before having my first son, I suffered two miscarriages. I started to bleed heavily with my second son and I thought FOR SURE I was miscarrying. I was sad, but a part of me was ok with it. I figured it wasn't meant to be and blah blah blah. We'll, when I went to the dr and heard his heartbeat loud and strong, I knew I wanted to do whatever I could to have this baby.
I did, and he's 6 months now. Sure, it's harder with two. Yea, it's more expensive and I'm tired. I work full time so I have that added on my shoulders. But, it is absolutely awesome to have two little guys. To watch them interact and play and laugh at each other. I look at my boys and I can't imagine it any other way. The things I stressed about SO much are nothing. Our family adapted and we couldn't be happier. There were sacrifices, as there are when you decide to become a Mom/Dad. But not as much as a shock as having #1 was! (Like another poster mentioned/)
As for school, take a year off. Seriously. Enjoy being pregnant and being a new Mom (again). Enjoy the moments when your new baby will smile at his/her older sister and enjoy all the hugs and kisses your older one will give the new baby. School can wait...these moments don't.
Good luck. I truly wish you the best. :)
T.