I feel the same way. I know that I feel the need to please others so that they won't think I'm a bad person. I find that my automatic response to anyone asking me anything even to the point of it being before they finish asking is to say 'yes'. What I have tried to do to keep myself from automatically saying yes is to try and stop myself and instead say, "let me think about it". This gives me time to either think about it right there (if it is something I want to do,) ask more questions (to learn what I would actually be agreeing to), or to go away and come back at another time with an answer. I find it much easier to say no then. It keeps me from saying yes automatically and if I find it is something I don't want to do (or shouldn't do for whatever reason) I can come up with a reason to give the person. When I started I often still found myself answering 'yes' when I came back because of my fear of disappointing the other person but I am slowly being able to say 'no' more often to things I don't want to do or shouldn't do.

It is always important to remember to take care of yourself. One thing to expect is that people may not react well initially to you saying 'no' since they are used to you saying 'yes'. But, they will get over it. That is their problem, not your problem.

I'm sorry if I repeated things from other posts since I didn't get to read all of them.