Don't overexplain your no. There is no need to explain yourself when saying yes. Why would you have to when saying no? Sometimes we may think that we need to explain a negative decision more than a positive one. Most times, societal conditioning, expectation by others, not wanting to disappoint someone and many more reasons can lead to us feeling guilty when making a decision to reject something or someone.
This often leads to insecurity and doubt if the decision was really the right one or if we should reconsider. There really is no reason to overexplain a no. So stop doing it. Setting boundaries can work wonders for your energetic sovereignty and put you back into the driver's seat of your life.