whY am i like this

so.. I'm 18, and when I get PMS i become a monster. i get so angry and irritable that I become an absolute nightmare for the people close to me. none of my past relationships exceeded the two month mark because once they have to deal with me during That time.... oh boy.... but rn I've been in a relationship for approx 3 months and I am Very afraid of scaring her off !! i warned her straight up that I would be insufferable but idk how bad its gonna get ! i fear !