From experience at Bristol, i can't talk from other universities' perspectives, they break you in realllll slow. Our first DR session was on the the ethics of it all, and then after making sure everyone was ok with the idea of seeing a dead person disected, we saw a couple of specimens. Anyone feeling ill or upset etc could sit down , or leave the room, or chat with the demonstrators. TBH, you very quickly see it as anatomy, something to study, than it being a dead human being who had feelings etc.
I can understand your anxiety but it will be ok over time, dont suffer in silence tho when you arrive, do let a demonstrator know, they will be really good about it.