Amy, here is something I wish someone would have told me before I had my second child and went through a tremendous amount of guilt. There is a likelyhood that you will go through a period where you might feel like you don't love your firstborn as much as you used to. I had this after my second was born, though now I can't remember when it started. I really felt like I may have made a mistake by having another when I started to feel less emotion for my first child. It was awful. I didn't feel as connected to him, he would get on my nerves so easy, and I just didn't have that "feeling" for him that I had before, iykwim. It lasted about six weeks or so and then it was gone, and I loved him more than ever once again.
After it was over I asked some of my other friends if they had felt that way, and when I described it they would look at me with amazement and say "YES!!!!" They just never admitted it to anyone because they felt so bad about it. It's like it's a dirty little secret. Thankfully, I am one of those people who figures I can't possibly be the only person who feels the way I feel, so I feel other people out and ask questions. It always turns out that moms are so relieved and willing to talk to me. It's the same way with my depression. I know I'm not alone, though I may be the most willing to talk about it.
I just wanted to share my experience in case you start feeling the same way, and feeling really guilty about it.
BTW, congrats on the new baby.