Do you have trouble saying "No" to people? Most of us desire love and approval, and that can make turning someone down difficult. Yet, saying "No" is a way of taking care of yourself. And as we know, self-care is a precursor to care and compassion for others. It's tough to give to others from an empty well.
When you do say "No" to others, do you feel guilty? Yup, I've been there and done that, too. It helps, however, if when you feel that guilt, you ask yourself some questions. Did that other person have a right to ask you to do something? Yes. Did you have a right to say either "Yes" or "No?" Yes. If you had said "Yes," what would have been the outcome for you? And that's a very important question. Because often the outcome is that you feel angry with yourself for doing something you don't want to do or don't have time to do. You are upset because it takes you away from things you want and need to do. You begin to feel resentful. You also feel somewhat dishonest and inauthentic because this really isn't what you want to do.
It helps to realize that some things are yours to do while other tasks are really for others with the skills, time or desire.
Remember, too, you don't need to offer an explanation if you don't wish to do so. "No" is a complete sentence. You don't need to give reasons or excuses. Assertive communication is honest and is part of respectful relationships.