Well since you are in AIBU I will say yes, you are. Sadly not everyone has been given the same chances at education, neither is everyone as able to learn or as intelligent as those who have it all spot on.
For many years o thought it was could have - nobody taught me otherwise and in everyday speech that's what I heard (coming from could've obviously) so yea I think YABU.
I can be quite sensitive about these threads though; I think they are juts mocking people less educated and able which isn't a nice trait. I have had to work really hard to pick up a lot of the basics in written language and when people flippantly comment about how awful it is to see of instead of have, I do question their motive.