I've only really stopped one project after starting it, and that was because I just didn't have the time. But I've definitely had my doubts about a number of them, especially since I started working by myself. It was much easier to stay motivated with at least one other person sitting there helping and encouraging. I think I doubt more that other people will like my vids than whether they'll be good- because at least in my eyes, they will be

I was afraid people would watch Dark Metropolis and either completely not get it or shrug and say "Eh, he just recreated the Dark City trailer.", that my M:I Project segment would be the plainest and most boring segment and be completely out of place, and that I would get lynched for making and entering a Powerpuff Girls video (PPG:R) in an AMV contest.
It is sometimes hard to keep up your drive to make a video, though. Dark Metropolis was started sometime in August of last year- and not completed until nearly New Years. Sometimes it helps to just step back and give it a rest or work on something else (in my case, the M:I Project was just what I needed).