I don't think i had PND with DD but my baby blues lasted for a good few months. My friends talked of overwhelming love and protectiveness from day 1 but i didn't feel anything but panic and resentment (I didn't like my post-birth body, I was physically affected by the delivery and blamed DD I think). I felt like my life was over, and I wasnt prepared for the sleep exhaustion, and what that does to you. I felt like i couldn't cope, relax, think logically anymore.
When I spoke (in tears) to my GP at my 6 week check, saying I couldn't relax, she just said "that's normal" but i didn't feel normal. Luckily she made sure my HV visited me and I talked things through with her, and she suggested getting out more and meeting other mums. That way I realised I was doing OK really, and could share experiences and feelings. I didn't know about mumsnet then, but think it is brilliant for the support network it provides. Sorry to waffle on.
Only DH knows how I felt in those early months with DD so it is quite emotional to think about it again. Luckily I discussed how I felt with DD with my HV before I had DS and she was really good and said if i start to feel the same again to let her know. I made sure I accepted offers of help from family and friends second-time round, and my elective c-section with DS was so wonderful, I bonded straightaway.
I still feel closer to DS than DD but not sure if that is to do with early bonding issues, or whether I feel guilty still over how i was in those early days. I feel like I have missed something.
I guess what I'm trying to say is 1) speak to professionals if you think you are struggling 2) take offers of help and dont feel a failure if you need to ask for help.