I actually did have to think about this a little, because every part of the writing process has it’s difficult moments for me (certainly I can get angsty and worried about every part of it!), though overall I do find early drafts harder than the revising process that kicks in as I’m writing later ones–revision is often where I really begin having fun.
But I think the one consistently hard thing, at least for novels, is simply how much time is involved in making one. No matter how hard any particular part of the process is, I don’t think it would be that hard if you were only there for a few days … but often, when a particular part of the process is being difficult, I have to deal with that difficult-ness for weeks or months … and doing the same hard thing day after day is harder than just pushing quickly past and moving on.
Does that sort of make sense?
What do the rest of you consider hardest about writing?