I think Easeful has a good point. We spent our lives being told we were useless/hopeless/losers/couldn't do anything right.
So we still have those tapes that run through our heads. We have one of two choices - go the perfectionist route and drive ourselves insane trying to be the 'ideal' (which was the path I went down - great for the career, crap for your stress levels), or never finish anything - if it isn't finished, it can't be judged.
My husband has a horrible time finishing things. He's getting his MBA right now, and honestly, he'll finish a project/paper, then be totally down on himself that he didn't do it well enough/could have done it better etc. What I don't understand in him is where it comes from. He is not an ACoA, and comes from a family who was largely supportive of him in whatever he did. But that one trait of ACoAs he's got in spades.
He tells me he doesn't like to say he's finished a project because he knows he'll always look at it and think "Oh, I could have done this to make it better".
The flip side is that he helps me a lot with my perfectionist tendencies (why clean the sink til it sparkles when you could be goofing off?) and I help him with his inability to finish tasks (nicely, not nagging). Together we make a good team.