Wow! You sound like me. I do things for people sometimes because I'm scared to say no. It's not something that you can just change overnight. You have to go about changing it slowly. Say no to some of the smaller things first and then it will become easier to say no to bigger things. It's certainly very painful stuff when you start to say no, but once you get in the hang of it the guilt about saying no seems to fade a bit.
Try saying no a little at first and maybe only on one person in your life because you certainly don't want everyone you know getting hot headed all at once. Cos lets face it, some people will be angry when you say no. That's the whole reason why we say yes. We're scared of their reaction. So you really only want one person angry at a time, if that makes any sense. They might not get angry, you might be surprised. I had to say no to this guy who wanted to have a relationship with me and i was so afraid. But he was thanking me for telling him and he respected me heaps more after that. people respect you if you let them know their boundaries. I am no expert on this, I just know what has worked for me. I am the queen of people pleasing.