tl;dr: I was with my LTR of over a year in a group setting, she was in a bad mood and did something slightly bitchy (passive aggressive). I called her out on it, pretty harshly, and she became extremely upset. I tried to remain firm but she was completely disgusted with me, I'd never seen her like that before. Ignoring me and making it obvious she was pissed at me. Since it was a group setting and we had to be in the same place together for the rest of the day, I didn't want the tension to fuck with the vibe so I eventually went partial-beta so she would behave normally.

Looking back I don't know what I could have done differently. I feel if I stood firm by my words it would have ended with her acting like a child the rest of the day and people starting to side with her against me.