I have the same problem: 'yes' is so much easier to say than 'no,' but the quantity of my projects is affecting the quality, and I don't like it.

Derek Sivers has a suggested approach for over-committers: "Hell yeah, or no." It's helped me cut down on my projects (and leaves me with more awesome things to work on, on average). Give it a read: http://sivers.org/hellyeah