Thank you for sharing your learnings about this here. Saying no can be quite a challenge when we are used to saying yes.
Maybe of interest/use to you also; One of my previous students taught me (and she made a podcast including it) that she now just stops sometimes, mid-conversation or mid-whatever she is doing, when she can feel some overwhelm approaching. She says “I just need to take a minute” or similar, and she then simply stops and regroups her attention, without the need to say or physically do anything. She pauses. She gets herself together and then she does what she wants to afterwards.
I have been trying this at times and sometimes find it is the only thing that helps at that time. It feels a funny thing to say midconversation, but less funny for the other person, than me waffling on to them when I am grasping for ideas/words etc. It helps me to think clearly when asked if I will do something, so time to gather the “no” perhaps. Taking a minute is a new helpful tool for me at least.