Setting boundaries and saying "No" aren't always easy. This is particularly true for women, who seem to be the caregivers and caretakers of the world.
It's good to ask ourselves what price we pay when we say "Yes" too much. Exhaustion and high stress can be part of the cost. Resentment surely enters the picture. We may feel anger at ourselves because we're doing things we don't want to do. We may even feel dishonest because we're not telling the truth when we say "Yes, I'd love to help out."
There's a cost to saying "No" but it's one we can move beyond. When we say "No," we may feel guilt afterward. But in time, we can let go of that as a consequence. Remember that "No" really is a complete sentence, too. You do not have to offer an explanation of why you can't fulfill the other person's request. And definitely don't apologize. It simply isn't necessary. You do have a right to refuse a request, after all.
Rather than just saying "No," a friend of mine says, "That isn't my cup of tea." She offers no other explanation, and she says that's generally accepted without any questions asked. That's another option.
Practice this behavior until it becomes easier for you to "just say 'no'." Boundaries are really important to our health and well-being. It's a matter of self-care.