I think if more mothers admitted how hard they found it, birth rates would fall massively. 
I feel your pain alltheseasons. I was a single parent to twins and while I loved and felt protective towards my DTs, I didn't have PND or anything, and there were many 'special moments' which I cherished, my over-riding feeling of the first 18 months was that it was simply bloody hard work! I resented the feeling that my identity was subsumed my motherhood, I resented the fact that I couldn't even go out to get milk without planing a major expedition, I resented the loss of spontaneity, the absence of a social life, the sleep deprivation (not because they were bad sleepers so much, but because looking after small children requires so much it leaves little time to do anything else and sleep has to go).
The only thing I can say to you is that it DOES get easier. 18 months was a turning point for me, but they got a lot easier after 12 months TBH. Things I found that helped was adopting a routine (made me more efficient and gave me back a few hours a day, plus the DTs thrived under it and became easier to manage), and forward-planning (e.g. meal plans, working out what you need to get if you're popping out for something so that you only need to do one trip, etc).
What support network do you have? Is your OH helpful? Family? It's vital you see other adults who you can lean on. You're an individual, not just a mother, and your needs need meeting too.