I have always had trouble finishing things, but it is more out of a fear of judgement of others. I hesitate all the time, daydream, get distracted, do ANYTHING else besides the thing I need to work on. If I need to grade, I will clean. If I need to clean, I will check email, if I need to check email I will rearrange recyclables. And on and on.
I have a theory that a lot of ACOA behaviors are somehow underlain by a need to distract oneself from the 'elephant in the room' that is the alcoholism/addiction of a family member, or just the screaming and threats that happened/will happen/are happening. One gets good at not dealing with the biggest problem in one's life.
Bragi, I haven't figured this out myself. Your insight that a fear of running out of things to do may well be, as noted by GingerM, a need for perfectionism. Believe me, it is a really common trait in alcoholic homes. Something is driving you, or leading you, away from finishing things. The best answer may not be to find the ultimate cause, but to use a cognitive behavior type solution. By this I mean that you do not try to find the cause, you 'talk back' to the emotions (fear) that push/pull you away.
I am not sure this is helpful, but this is a shared problem.
Off to finish something I started over an hour ago.