Why it's sometimes hard to stay focused

As I wrote previously in a post about "Getting Things Done", I'm struggling with trying to define a system or work pattern that helps me to finish things and not get so caught up in distractions. I picked up David Allen's book, and while I've just started reading it, there is one quote in the first chapter that kind of sums up the problem. After writing that in the old days, knowing what you needed to do to get a job done was self-evident (getting a field plowed, working on an assembly line, etc.). When you were done you were done. Now in many professional jobs, and I lump basic science into this, it's harder to figure out what you need to do today, do it, and coming away with a feeling like you accomplished something.

"...for many of us, there are no edges to most of our projects. Most people I now have at least half a dozen things they're trying to achieve right now (my emphasis), and even if they had the rest of their lives to try, they wouldn't be able to finish these to perfection."

I think recognizing this is one step towards at least feeling better about what happens (or doesn't happen) in any given day.