The simplest solution I know of is to start raising your prices. You will gradually start to get fewer new clients, and gradually start to lose a few of the old ones, but individually the remaining projects will be more profitable and enjoyable, and the work volume will become more manageable, all without having to ditch any of your clients (rather, they will be ditching you, leaving you off the hook).

A book you might find useful is 'The Business Side of Creativity': http://books.wwnorton.com/books/detail.aspx?ID=4294978649