saying "NO!"

So, I have a small problem when saying no. I end up feeling guilty and saying yes. Lately, I've been listening to Peter and have been saying no to the things in my life which I don't enjoy and don't need to continue. I tried that with the Yearbook. However, I was not ready to have to say no a second time, to the same question. It makes it so much harder to say no a second time. I have no idea why, I just feel badly because they can't find anyone else.

Is it a good reason to do something just because you'll feel guilty if you don't? I can finally say no, but saying no again is harder.