Your trainer is giving you a great piece of advice! This has been a real "eye opening" experience for me personally, to say the least. Best I've been able to gather from my trainer is that as she watched my horse becoming more and more terrified, she was wondering to herself whether she should pull us from the lesson. My only guess is that, just like me, she did not want to cause a "scene" in a clinic with a BNT in front of all the other local professionals... Thankfully, both of us were smart enough to realize that there was no way to justify going back for a second day of that insanity. Afterwards, with positive encouragement, it took us 4 days to get my horse back to being relaxed and happy in the ring, instead of constantly worrying about being punished at any moment for no reason at all...
My barn manager had the same type of experience with AK, too. Her poor little mare was just fried afterwards and she said that she wished she would have just left, but there's that feeling of, "Wait, this is a BNT and I'm just a minion...she HAS to know more than *I* do, right?"
I've never cliniced with anyone before, but reading so many of these kinds of threads gives me pause. Auditing, sure, but actually taking my horse to someone now makes me nervous! My trainer has actually advised me to be very cautious about who I clinic with, because my horse is kind of a weirdo in how he likes to be ridden. She said that if I'm ever in a situation where the clinician is telling me to ride him in a way that is counterproductive, I'm supposed to either fake it or walk out.