I think everyone has different things that they struggle with. For me, I can pretty much power through the sleep deprivation and nappy changes don't really worry me too much (and DD currently does 4 poos a day).

What I do struggle greatly with is:

Never having time to myself (10 minutes would be cause for a happy dance)
Constant, constant interruptions so feeling like I can never get something done from start to finish without being interrupted whether it's something like loading the dishwasher to actually finishing a thought
Not having any quality time with DP (he's a shiftworker so usually works nights) so all our time together is spent with our DDs
Having to choose between putting the girls in childcare or taking a career break. Professional part-time work is quite difficult to find and employers are often gooses when it comes to being open to the idea of two professional women job-sharing.
The impact that being a SAHM has on your relationship with your partner. It feels sometimes, like I have made all the sacrifices in terms of the above and especially with regard to career while DP's life has remained pretty much the same. It often appears to me that the couples who have the best relationships are where the mum has gone back to work full-time and the child-rearing is shared equally. That's just an observation of my closest friends. It feels very difficult to balance the girls' needs, my own needs and the needs of my relationship.