Assertiveness
Assertiveness is stating what you need or where you stand, clearly and without aggression. It sits between staying silent to keep the peace and steamrolling the other person.
Assertiveness sounds like I would rather not, or I need more time, or that does not work for me, said plainly and left to stand. It is not a raised voice and it is not an apology wrapped around a request. You say the thing once and you let it be true.
For people who please, assertiveness can feel like aggression, because any expression of your own need once carried risk. Speaking up might have brought conflict or withdrawal from someone you depended on, so silence became the safer move. The discomfort you feel saying a clear sentence is that old alarm, not a sign you are doing something wrong.
Assertiveness does not require the other person to like your answer. A clear no with no explanation attached is still kind. You can say it warmly and still mean it.