Bless you OP. It’s so hard having a miserable baby. DS was just the most discontented, demanding baby ever. Months 4-6 were particularly bad as were 10-14 ish. I feel like all he did during those months (/the whole of his first year) were whine, fuss and cry. I too was utterly depressed but it was all secondary to how difficult he was, not PND.
Firstly, does she literally cry all day (I mean every single second with no let up) or is it more situational? DS cried a lot, far far more than average, and for hours on end…but he did have periods of being happy - usually forward facing in the sling and being walked round outside in a busy environment, or at a baby sensory class, for example (although he screamed blue murder in the car en route there and back). If she literally never stops crying, I’d be getting a second medical opinion. If it’s situational, she probably is just really really high needs. I’m sure you’ve probably come across this concept when googling before.
You probably already know this, but if it is ‘just high needs’ then she will grow out of it. DS (just over 2) is now a massive extrovert, fiercely independent and requires vast amounts of physical and mental stimulation to sleep…but with every milestone he’s got so much more cheerful, to the extent I’d actually say he’s a happier than average toddler now. No tantrums or screaming rages or anything like that so far.
My only advice to survive it would be, don’t be afraid to use formal childcare (if you can afford it) to give yourself a break and to give you some time with your 3 year old. A couple of mornings a week or something like that when she’s 6 months old. My miserable baby swallowed me up and consumed me and I only really found myself again when I went back to work at 11 months.
Good luck x