Sometimes you need to say 'no' to even your best friend, or lover. I used to be a total failure in those cases. Though I am not the person who tries to please everyone, to get her way through; I am afraid of hurting anybody, even if I actually detest that person. That is really a weakness.
Saying 'no' as a matter of principle, particularly in professional dealings were not difficult for me, I guess because, there was always a logical understanding that what I was to effect others, and not only one. But when it came to choosing between hurting me and another, I buckled.
Later, I discovered that stalling the decision for some time, and later taking it up, to state clearly why I don't agree to it, is a better way of saying 'no'. It may displease the other person, but cannot hurt him, if the relationship is meaningful.