When I am around other writers, they sometimes will talk about having "writer's block" or experiencing difficulty writing. I feel their pain, having experienced this from time to time myself. I heard a writer once answer the question, "What do you do about writer's block?" He said, "I lower my standards and keep on going!" He went on to say that he could always revise what he had written when he got his mojo back (my words, not his--he said, "When I'm able to write well again").
I think a key is also to write at the same time, in the same place if you can do it. I've known writers who are still working at their day jobs who get up early (4 AM for example) and put in three or four hours before heading to work. Others who can't manage a block of time (like me, even though I am retired) will write whenever and wherever they can. That's what I tend to do although most of my writing is done on our desktop in the "computer room" (AKA the glass-enclosed nerve center of the Biscuit City Network--a fiction of my mind, I have to admit). If I can't use that, I have a laptop and email the files to myself so I can have them on the desktop. I don't like to carry a laptop around when I'm running errands, so I will end up putting down ideas or phrases or even paragraphs on notecards if I have one or scraps of paper if one is available or my hand (the original Palm Pilot--ar, ar). I could carry my writer's notebook but I'm afraid of losing that after I misplaced it for a month because I took it to church and left it there and forgot about it.
I've also talked recently with another writer about the usefulness of deadlines to move the process along. I along with others always want to change what I've done, fiddle with it, make it better. A deadline puts a stop to that, although deadlines do result in some late nights and close calls. I do have a deadline for my Observer column although my editor is very understanding. I still try to respect deadlines so I don't inconvenience the publication which has to meet a publishing deadline.
And if I want to change something that has been published, I can always put a "director's cut" here. I can do that because I write short pieces that will fit into a blog space, although I don't really know how long these posts can be. I haven't written pieces that are that long. It is possible to re-do a book, but you have to be Stephen King or someone like that to do so.
One author said it well about finishing a work when he remarked, "I never finish a piece: I simply give up on it."
So good luck to all those of us trying to finish pieces and meet deadlines. I hope you're able to do so, and that you don't have to give up on them! Keep writing!