I've read that Leonardo da Vinci took many years to complete some of his work. I find that sometimes it's good to get away from a piece for a while and get your mind on something else. When you come back to it you may find a new perspective, as if you were previously working with "blinders" on. Of course, when one gets a new idea or golden opportunity, one must pursue it even if you're presently working on something else. You can always come back to what you're "finishing" but if you don't pursue opportunities as they arrive, they will pass you by without returning. New ones may arrive but the ones that have passed, are gone forever.
The same thing happens in my photography. I will usually upload a days worth of shooting into a folder on the computer. I will try and select the the best of the batch for processing first. I cannot process all of them in one day and in the meantime I've shot more photos and uploaded them into another folder. After a while, I have many folders with images needing to be processed. During the good weather months, you have more opportunities for shooting. In the cold months, I find myself inside more and processing images that I've uploaded previously.
Overall, I think the best work will eventually be finished while the lesser works may or may not be completed. I can't really say this is a bad thing, but more of a natural filtering process which hopefully puts my BEST work forward.