I just saw your post from the other day and I just wanted to tell you that I also had some depression throughout my first pregnancy, even though I was overjoyed to be pregnant. It's just a very emotional time, and I'm sure for you trying to handle all those emotions without the support of a partner is even more challenging. In my case, once I had my DS, things got much better. The first month or so was tough while I figured out what I was doing but after that, it was wonderful. I was madly in love with my baby and I felt truly fulfilled by becoming a mother. I know you are feeling anxious about having a second, etc., but I think you will be so in love (and so distracted!) when your little one gets here that you will be surprised to feel much better emotionally, and even more secure. I know that's not the case for everyone, but it definitely was that way for me, and for several of the other new moms on this board.
Good luck to you and take care - I am pulling for you!