Sorry if my advice about just doing whatever you want to do but taking the baby sounded crap - a subsequent poster said it never made sense to her when people said that - but I dont mean your daily routine as such, its more a case of a random stranger (me) giving you "permission" to just do it anyway.
Not that you need it - but maybe you do?
This is sort of separate from the PND thing - I know the PND makes it difficult for you to do just that - but even if you dont end up doing it, try and get in the mindset by thinking, "how could I do that with a baby? Would I bring a sling? Would I time it with their nap?" and try and strategise scenarios in your head. It makes you feel freer when you do finally achieve these outings and events with baby in tow.
I have had my legs waxed with a 2 and a half year old and a 6 month old in the room with me. It wasn't my idea, the salon lady said "just bring them" when she couldn't accomodate my child-free time and I said "oh no you have no idea" and she said "there's no one else here, what's the worst thing that could happen"?
Somehow it worked out fine. I have had my eyebrows threaded whilst my 11 month old shrieked so loudly the walls of Fenwick nearly fell. How was I to know she was going to object? Nobody died (though people wished I was dead, probably)
Small steps though and I appreciate that its easy to say do it. Just think about it for now. Allow yourself to imagine walking round somewhere or doing something or other even if its when you are dropping off to sleep at night...just visualise it if you can.
I know that trapped feeling. I had it. I didn't have PND, or if I did, it was short lived and mild, but I had that depressed, trapped feeling of the walls closing in on me yet not having the motivation to get out (or, some days, get dressed). You feel worse for it somehow, its a hard cycle to break.
The babies get to an age (around 7 months or so can't remember) where you almost have to go out each day or it can just go a bit wrong! Even to this day, our one day at home where nobody is at preschool or anything like that, on days where I decide we dont need to go out etc, god it drags and I'm talking through gritted teeth to my son by 1pm most the time.
You just have to get out, even if its for an hour in the morning. Do you drink coffee? Even if you just go to a coffee place, order a coffee in a takeaway cup, and sit at a table to drink it for as long as you dare, allowing yourself to leave with it and drink it in the car/way home if things go wrong.
Sorry it all sounds trite really whatever advice but if one little part is helpful then I'm glad.