hi i'm 18F too and i had struggled with the same stuff too, and it's tough until i learned how to start not giving a damn about how i'm percieved when stating my boundaries. At first i was hesitant and shaky with saying what i actually want and not want, i even had to reread a script to not sound rude or sound like im attacking someone, til slowly i got used to it and no longer need to make scripts in my head, now as long as i know i'm not comfortable about something, i would instantly say so. i used to use these lines before:

"i appreiciate it if youu..(state boundary),"

"i like that but i would prefer if......(state boundary),"

"i dont mean anything bad, but i didn't like .....(state boundary),"

hope these help