I'm pretty saddened by how many similar experiences many of you have had, but I'm glad to know that all of you have kept creating anyway! (Spite can make for an excellent motivator.)
I wouldn't call this the MOST demotivating experience I've ever had, but this is the first one that jumps into memory.
Some years ago, friend found out through another friend that I was writing a book. I wasn't all that interested in sharing the details with him, as he could be pretty opinionated and harsh about pretty much everything.
"So, can you tell me what it's about?"
"Just some fantasy and sci-fi stuff. You probably wouldn't be interested."
"Oh come on, talk to me about it."
"Eh, I'd rather not right now."
"Oh, well how about I talk about a book that I've been reading, and then you tell me about yours."
"Whatever."
He proceeded to spend about 45 minutes (I kept checking the clock, actually) talking about this extremely depressing novel he was reading (A Little Life, specifically), before then asking me to talk about my work again.
I got about two sentences in before he suggested that I find an editor and promptly changed the subject.
I was mostly angry at how rude the whole exchange was, but it was also a bit of a hit to my self confidence when I was already struggling and feeling pretty bad about my work at the time.