Taken for granted!

Everytime I swear to myself, "this is it! I am not to let myself taken for granted again!" and curse that I allowed myself to be in such a position. But everytime I soften down! On many occasions, I can not say 'no'! 'NO' must be toughest word to say! I understand that being nice to others is not really being nice to myself sometimes. Also I can not stay angry at someone or at something for long! What can be done? I wish I could just give a 'damn' sometimes!