I suspect the problem isn't so much the lack of hours in the day, but the ability to "see" your work and know what you're capable of handling so that you can actively make decisions about what to take on and what to defer.

In agile/lean circles we call this "limiting WIP" or work-in-progress. As human beings we have a finite WIP capacity, and we benefit from keeping it low so that we complete what we start (which has a powerful psychological effect). One way of doing this is to set up a personal kanban board so that you can visualize your work and control how much gets into your WIP queue, worked on and then completed.

A fantastic, easy-to-read resource for this is Jim Benson and Tonianne DeMaria Barry's book, Personal Kanban: Mapping Work, Navigating Life[1]. It quickly explains the theory behind the technique (adapted from lean manufacturing techniques) and how it can be readily applied in a variety of contexts.

Nearly all customers I've coached to adopt this technique have benefited from it as it leads to a deeper understanding of the work they do and how they do it.

Good luck!

[1] http://www.amazon.ca/Personal-Kanban-Mapping-Work-Navigating/dp/1453802266