There are people who have a hard time saying "No!" These people either find themselves working beyond their capacity and find themselves overwhelmed, or they are the type of person that backs out at the last minute when they find that they can't fulfill their obligation. Saying "No" may seem selfish or "mean" but saying "No" is actually helpful and responsible. Very few people enjoy when someone either backs out of a promise to help or they do the job but at a sub-par quality due to being overwhelmed. Saying "No" can not only help with your own sanity but also with your relationships with others. "Under promise and over deliver" is a good rule of thumb. It is better to say "No" or even "Maybe" and the come through and help than to back out from a promise when someone is counting on you.