A solid, stick-the-landing "No." takes practice.
Start small. Let it be about unimportant things first, then work your way up to the very vital. Let people get a little miffed. Soon enough, they will see that you are changing your "always YES and more YES" patterns. They will adjust.
Later, when it really matters, you'll understand what it takes to stand firm in your firm refusals. You will also learn that the responses you get are about the responder, not you.
You will change and others will have no choice but to get over it.