There are far too many good things to do. We are incapable of doing them all. Most of us pack too much into our lives. When we do too many things we enjoy them less, we do them poorly and we experience unhealthy stress.
When new opportunities come up we tend to ask ourselves, "Is this a good opportunity?" We reason to ourselves that if it is good opportunity we ought to take it. If our plate is already full, a better question would be, "Is this opportunity better than one of the things on my plate right now?" If it is, we can drop something in order to take the new thing on. If it is not, we can release ourselves from the guilt of saying no to the new opportunity and move on.
We probably need a "to don't" list more than we need a "to do" list.