I am too NICE!!

Man, I think I just have a problem with saying no to people. Even though I really wanted to finish work on the 15th, my boss asked me if I could work another week. I really didn't want to, so I kindof came up with the excuse that I needed to go house shopping (which I still need to do). But anyways, I agreed to work until Wednesday *sigh*. I can't say no, but at least I can compromise to a shorter extra working period in my head. MAN! And for the SAME amount too!! Maybe I just don't know how to say no without sounding really selfish or mean...I guess it's something that I need to work on.