the boundary could be imposed on you (ie: you leave the room), the boundary could be imposed on the other person (ie: you tell them how you feel and tell them the consequences

I was in the same situation as the OP and I have done both the things you have suggested and guess what they didn’t change anything. Yes the day I told them what I felt and walked out of the room, felt reliving, as I did what I wanted to do and the next few days felt different too. But things came back to being bad as they were before.

And this made me think that, I shouldn’t have been the nice/doormat guy I was from the beginning itself and be more open about my views and opinions which would make me look like a strong character. First impression is everything and you really can’t do anything after they’re set.