Every talented woman I know - and quite a few fabulous men as well - struggle with how and when to say "no." The reasons why this is are well documented, but instead of getting stuck in the mother's hood on this one, let's step out.
I have a catchy new way to say "no."
The idea is simple. When you slow down enough to check in with how you feel, and you "know" the best answer for you is "no," say, "know."
Your answer, "know," will automatically convey the grace and thoughtfulness you desire to show, not the shame, disappointment and judgement you fear you will attract by setting a clear boundary.
This is easier said than done, but if you can be mindful of your intention - to convey sincerity and grace and thoughtful concern for the human being asking something of you - your "know" will only be an act of self love, never a rejection of another person or idea, so check the guilt at the door.
P.S. Those to whom you are saying "know" still may not like it much, but you will be less involved in what they are feeling, so it shouldn't bother you much.