It's the PND and the whole sense of being overwhelmed with a new baby that makes everything seem like it says something horrible about your life. Under different circumstances, in a different state of health, this might have been a hugely funny experience, something you looked back fondly with memories of shared laughter and gallows humour and bonding over something horrible. But you simply cannot do that with depression: it's like trying to dance with a broken leg.
Things may feel a little bit better with a different narrative, though. Not because the "broken leg" will magically heal, but at least it might help to stop beating yourself up about your dancing ability. How about instead of "we were to have a magical holiday and we didn't, but somebody else did, so that means we are failures" trying to tell yourself "this has been a tough start to parenting, but we are fighting on, we are not giving up, look how far we have come from that time"? It's the same story, just a different angle.
That is what your baby deserves: a mother who looking back has to admit (as you did) that you have come a long way, a mother who is prepared to consider ways of fighting still harder (by seeing a doctor or doing whatever it takes)- in short, a mother exactly like you.
My dd is now a young adult. In many ways, she has had a shit life: she has a chronic pain disorder, she suffers from some MH problems, she has suffered suspicion and investigation from school authorities and Social Services, she has even tried to take her own life: on so many levels we have not been able to give her or her brother the golden carefree childhood they had deserved. But we have hung in there. And they have had a better life than they would have if we hadn't. Sometimes that is the best you can do. And their life has not been totally full of misery: we have had good laughs, we have good memories, and (I hope) they have a feeling that whatever happens we will be there for them.