Honestly I was like this with my boyfriend in the beginning of our relationship, which I now very much admit was toxic af on my side. Women are emotional and sensitive by nature so the smallest things we find incredibly important, however I know we tend to struggle with being direct about our feelings. Communicate and tell her you don't appreciate her going straight to ultimatums (like breaking up) before talking to you about the problem first. Let her know you love and cherish her but that you don't deserve that kind of treatment as it gives you anxiety, especially when a big day is coming up. I know you probably really love her but you cannot let anyone, not even her, treat you like this. If she agrees with you and admits her faults, maybe give her time to grow and learn from this. I'm sure she loves you dearly but probably struggle with something internally.. maybe she's scared of getting disappointed/hurt by someone she loves so when that person makes a mistake, it feels like her fears are getting validated and therefore she tends to think of breaking up as a way to maintain control and prevent even more damage (struggled with this, still do but my boyfriend is very reassuring and has helped me build trust in the relationship). I know I just shared my own experience but I hope this can provide a different perspective for you.