I felt compelled to share this with you after reading this blog post: Dear Friend: Sorry. My heart says yes, but my schedule says no.
I hope you go and read his post after you read this. It is important to know what your limits are. I know we like to think that we can do everything. The reality is that there are only so many hours in a day. If you do not feel like you can give a certain task or project your best effort, you would be better off declining it. This keeps you from feeling overwhelmed and stressed all of the time.