Here are some quotes from Gordon T. Smith's book Courage and Calling that helped me think through why it's often hard for me to say 'no'.
The freedom to accept our limitations is evidenced in the ability to say 'no'. Many people experience life as one continual burden simply because they are trying to do too much. We accept more than we can possibly do well; we respond to the requests or needs of others, knowing that we are being driven not by 'a spirit of power and of love and of self-discipline' (2 Tim 1:7), but by some other propensity.
It it helpful to discern what prevents us from saying no. Some of us fear rejection, and our longing for acceptance drives us to do more than we should attempt to do. Others among us long for a sense of importance. If we are busy, if we are doing many things for many people, we feel worthwhile, important, and in control; we feel needed. Still others simply do not know what they are being called to do, and they seem to think that if they do as much as possible they will hit upon what it is that they should be doing!
The inability to say no inevitably leads to frustration. We can never do enough to feel that we have done what is necessary to gain acceptance or feel important. It is hopeless. Our only hope is to come back to a clarity of purpose and call, a sense of who we are and of what we are being called to do in this place at this time. We need to come back to clarity about what is important. And there will be many days when regaining clarity is all that we do. But at least we did what was important.
On a different...but similar topic...
God often calls people to the obscure, the ordinary and the mundane. He accomplishes some of his most important work in the world through ordinary people doing ordinary things. We should not merely tolerate this ordinariness; rather we must embrace and even celebrate it. Some of us miss our vocation because we are looking for the heroic. We fail to accept the limitations of life and the fact that God's work through us is often in the small and the ordinary. We also fail to realize that the work we are called to do will often be very difficult and that things we accomplish will happen slowly and incrementally. We are not heroes; we are merely people who are doing our best in our day-to-day work.