The ability to say no protects the quality of your yes.

Every commitment you accept dilutes the ones you already have. If you cannot decline, then your yes stops meaning anything, since it is given automatically and backed by no real capacity. The value of an agreement comes from the fact that it was not guaranteed and that there is room behind it.

Practise refusing something small this week that you would normally accept. Notice that very little happens. That experience is what makes the larger refusals possible later.