You really just have to cut yourself some slack. It's hard I know. I have felt (sometimes still feel) the same way. It feels like you are being such a "burden" on people. But you have to realise that you are unwell (PND isn't "normal" - it's common, but not the normal happy baby experience that others get). You have to accept that you need help from people to get through, and just give yourself a bit of a break.
It's hard even admitting you need help, I know, and then you feel bad for getting it... ugh.
I was the same, determined not to medicate, but I really had to rely heaps on others (DH, mum, friends) because I couldn't cope without lots of help. I ended up on medication, and once things settled down after that, I was able to deal with everything MUCH better. I don't feel as guilty for being so demanding anymore, because I'm NOT as demanding. So the meds worked really well for me.
You really just need to cut yourself a big break. You have PND - the fact is that you need a hand up. It's okay to get a hand up. That's what families/friends/health professionals are there for. When you are able to, do nice things in return... keep reminding your DP, mum etc. that you really appreciate their support and that they are invaluable to you, while you are having a hard time. They will know their worth that way.
Hang in there Kat