Ok... I am STILL catching up on this story. But at least I got a good excuse why it's taking so long -> fuckwad.
Anyways, don't despair. Everyone feels like that sometimes. What I usually do is decide on how a story is going to end (not speaking about the one-shots I do because they tend to have little plot and I just try to through in an overkill of "funny" stuff) because that way you know on what to focus. I usually don't change the ending (often I even write the last chapter when I'm only at the beginning) but it happened often that I added something that I didn't think of first or changed details or even bigger parts of the story. But this is just what I do and it might not be very helpful to you.
Just don't pressure yourself because that never works. ;)
Good luck!
Author's response
Actually, that is really helpful. Now if only I could decide how I want it to end...