Midge, I actually cried the first day DS didn't cry when I dropped him off! I'd gone through months of the bottom lip quivering, or an all-out bawl that I could hear from outside (the lovely nursery staff always reassured me he stopped as soon as I got in the car), then this one magical day he didn't cry, i thought wow, my boy is all grown-up (17mo now). got to the car and got all paranoid - OMG, he doesn't need me anymore, he prefers the nursery staff etc.! - I too have PND, but with or without it, i think a lot of us going back to work go through the guilt thing for varying lengths of time. guilty that LO is in nursery, guilty you feel happy to have a wee without a toddler scrambling to get on your knee for a hug (as much as i love those hugs!), guilty that you have to leave work at a certain time or take time off at short notice...
DS does still cry somedays now, but not for long, and I love it when I pick him up and watch him playing with the other LOs before he sees me, he's really happy there and has come on so much.
the nursery staff have told me babies settle in much quicker the more days they do in a row, and the younger they start there, it just seems hard to have your last week without your DD around in the day. Maybe do a couple of things you can't do when you have DD to care for - long lunch with a friend, swim (ie, not a stand in one spot whilst looking after LO in pool for 30 min!), go to an afternoon film (lots of popcorn & icecream!)