i know "no means no" but nobody ever told me the exact words. what do you actually say in the moment

this is gonna sound stupid but hear me out. ive been in the lifestyle for almost 2 yrs now (f33, partner he 36, west coast, mostly house parties + 1 club we like) and i still freeze when its time to say no. like the THEORY i get. consent, autonomy, no is a full sentence, all of that. but when im standing in a hallway and someone i just spent an hour flirting w leans in and its just NOT clicking for me anymore... what do real ppl actually say. out loud. with their mouth. like i cant just go "NO" sharp and walk off, that turns a friendly room cold. but "im not feeling it" feels weak and ppl talk me back into it. i need actual phrases. things ppl have said to me that worked. things you've said that felt natural and didnt make the other person feel like garbage. oh and btw the times im asking abt arent like... aggressive situations, those are easier honestly bc the answer is leave. im talking abt the regular nice ppl situations where everyone is being polite and the no is the awkward part. those are the ones i fumble. also do you give a reason or no reason. bc if i give a reason ppl negotiate it ("oh just for a bit") and if i dont give a reason i feel rude. genuinely asking, no advice on confidence pls, just gimme the words