I'd been telling myself that one child shouldn't overwhelm me
Oh, it makes me so annoyed, this sentiment!
There is no 'should' about it.
Motherhood is hugely mis-sold.
It is hard work.
It is the biggest life change we are likely to ever go through.
We get little or no preparation/training for this Most Important Job.
Undervalued, by society and individuals.
The pay is shockingly crap 
It can be boring and stressful at the same time.
It is repetitive to the point of insanity.
Somehow, women 'having it all' has in reality morphed in to women 'doing it all' 
Somehow this Most Important Job is meant to be done to perfection AND with a smile on our face AND while looking great while also working, keeping house, looking after elderly relatives, maintaining a hobby etc etc.
I did not have PND, I have 4 children and I cannot tell you how much more enjoyable life is now that my youngest is 7.
I really did not like looking after babies/toddlers and I do now make a point of talking about that sentiment when the subject comes up.
Wanting children and eventually giving birth to one does not automatically turn us in to saints. But it does automatically radically change expectations placed on us, by family/partner/sociatey/ourselves.
OP, you have every right to feel displeasure with your situation.
THEN - accept that things are messy just now, lower your standards/get paid help (as funds allow)/get your DH to pull his weight/make a point of counting your blessing while acknowledging that, yes, this is all a bit crap.
Wanna what stopped me from running away? 
'This too shall change, this too shall change, this too shall change.... ' - aka The MN Mantra.