If someone invites you to dinner and you don’t have any other plans—and therefore no excuse besides you don’t feel like going—just say you can’t do it that night. You don’t have to tell your friend that you are sick and your dog passed away and your toilet overflowed. If you want a quiet night at home, take it. You don't need to justify that or apologize! People don’t necessarily want or need your excuses; they just want to know if you can meet up or if you're interested in the job or if you can do them a favor or not. Saying no to something, for whatever reason, is often enough.
Of course there are times when you have really misled someone by saying "yes" to them again and again, so explaining your reasons for your perceived change of heart is called for. Those conversations are never easy to have, especially if you are faced with the prospect of hurting someone you care about. But keep in mind—should you find yourself in such a situation—that not being honest with them (and with yourself) will only have increasingly negative ramifications later on.
Learning to say "no" when you authentically mean "no" is a life skill. For some people, it comes quite naturally. For others, it requires habitual practice and conscious use. It may not feel right at first, but it's necessary for living life truthfully and a skill that will promote good overall emotional health.
Still not convinced that "no" is a powerful word? Here's how the power of no can save your emotional life.