it's like our brains have just decided it's this incredibly dangerous / awful thing and we can't switch it off!
That's exactly what's happening (except it's your mind, not your brain).
Babies - as far as I know - aren't scared of being sick. They do it regularly, copiously and often without any warning (a big part of the reason I am childfree!!).
So it's somewhere between early childhood and now that you've developed this phobia. Your mind is not trying to ruin your life by going into overdrive trying to keep you away from vomiting - on the contrary it's trying to protect you because it believes that vomiting is really, really bad.
What could have caused that? Maybe you were in a parent's (or grandparents) brand new car and told them you felt sick and got shouted at - "DON'T THROW UP!!" Maybe you overhear someone saying "Did you hear about so-and-so? They got sick and died." Maybe you got sick in school and all your classmates laughed at you.
Whatever the incident(s) somehow your mind came to utterly believe that being sick was the worst thing in the world and that belief is still there now. It is a protection strategy so don't berate yourself - your mind is always working in your best interests; it just doesn't realise that that strategy is, well, wrong, or if not wrong, outdated and misunderstood.
That's where I come in 😊