I know how hard it is to say no.
Perhaps you could examine within yourself *why* you find it difficult to do so? I have found, for myself, that it was a deep seated desire to be LIKED by others, not to ruffle anyone's feathers, and never to be the Bad Guy (some people equate saying no with being "mean").
I have learned slooooooowly to say what I mean and mean what I say, and to stand up for what I want. I even practice in front of my mirror so that it eventually becomes natural for me to say "I'm sorry but I just can't".