I may have blogged in the past about "sagging middles," but that's honestly where I feel my main problem lies. When I, as a writer, get to the middle of the story and I know how I want it to end, I sometimes find myself losing interest in the story. Not that I won't finish it, I will, but I can't seem to drum up the same enthusiasm that I had for that story and those characters in the beginning of the book. That's when I want to do something drastic in the story. Sometimes that works, sometimes it doesn't.
In any event, I'm working hard to get over that problem of losing momentum. I try various writing exercises like writing that scene from another character's point of view or writing it in first person or yanking the entire scene out to see what happens to the story without it. Sometimes they work, sometimes they frustrate me no end, hence the million and one "beginnings that have no middle and no end."
Eventually I will overcome that issue and begin writing again. I'll get involved with the characters and the plot and I'll finish the damn book. After the editing comes the part that makes me want to hide under the bed...submitting it to a critique group. Usually the feedback is good, and if it's not good, it's extremely helpful...but it's still hard to sit there and just listen without "defending" your work.
In any event, the time has come to get back to writing so I can get out of the middle of the book and get to the end...and then begin again.