The yes comes before the thought
What stands out is the speed. Between the question and your yes there is no weighing up. That is not weakness, it is a practised reaction, and practised reactions are fast.
That is why the intention to say no more often so rarely helps. It arrives too late.
The sentence that buys you time
You do not need a no, you need a pause. I will look at it and come back to you by tomorrow. The question is answered without you having agreed.
The sentence looks unspectacular and still changes the most, because it interrupts the practised reaction.
A no needs no justification, but it needs a reason
The difference matters. For yourself you need a reason, otherwise it feels arbitrary. For the other person one sentence is enough.
That does not work this week is complete. Everything after that is negotiating material.
Why it feels wrong even when it was right
Expect that. The guilt is not a sign that the decision was wrong. It is the reaction to having done something differently than usual.
It gets smaller once you have made the experience a few times. Not because you get harder, but because your head gets new evidence.
What is enough this week
One no to something small. Not the big project, not your manager. Something where little is at stake. It is not about the effect, it is about the experience.