Things left undone can be clutter. We know they need to be done, but for some reason we don't feel the impetus to finish. It's exhausting to think about uncompleted tasks.
This weekend I was helping my girlfriend with some errands. She'd been wanting to drain her hot tub for awhile. She'd mention it weekly with some dread. Finally she asked me to help her and we went outside and took care of it. After we had the hose connected and the tub started draining, she said "This has been hanging over my head for three weeks and after all that, it only took 30 seconds."
It can help to know that while thinking about finishing is exhausting, the completing of tasks is energizing. The energy to complete a task is contained in the doing, not in the mulling.