In my case, I have no problems in writing anything, but sometimes I suffer the 'cut the crap' process. I love my crap
Emotionally, this thing about my cat.
Technically, the second version of my first book nearly broke me. It wasn't right, but I wanted it to be right so badly, and I kept pushing and pushing at it until I couldn't think straight. In the end, it all went in the bin.
The hardest, most agonizing thing about writing for me is naming characters and places. I'm not kidding. It takes me several weeks. I don't know why but names are important to me. They have to be perfect.
This. Even worse is the title of whatever I'm working on. It's one of my little authorial quirks, if the title isn't right then I won't be comfortable enough to write the story. Title is both King and Queen for me, it has to be perfect for whatever stage I'm at with the story.
Case in point, earlier today I looked up the latin word for vampire for the name of a scifi novel about vampires being a subspecies of humans. I checked and rechecked a half dozen times before I was satisfied with the translation. The damn thing is still in just the concept stage! It's a good title though, based on the scientific propensenity for naming things in latin, Homo Sapien Lamae. Vampire is Lamia in latin, and homo sapien is the species name for humans.
You'd probably be better off with Homo Lamæ; all species in the Homo genus are human, it's just that Sapiens are the only ones still kicking about. I'm not an expert, so Homo Sapiens Lamæ may well be valid
too, though.
Personal preference abounds.
On topic: the only thing I ever really balk at is writing cards for birthdays and stuff. At this point I generally write them in whichever language is tickling me at the moment and relish the chance for a little extra practice; people just write it off as me being eccentric(more likely weird).
Beginnings. The entirety of the first act is so hard for me to get right. There's just so much you have to do; introduce the main cast, setting, plot, themes, and so much more. And then on top of that you have to make it actually interesting to read (Meaning I can't just make a 'things you need to know for the interesting part of the story to make sense' list.).