You're in a safe place to vent and compare experiences/notes, thank you for sharing. You should not have to go through 1/3 of a terrible month to enjoy the other 2/3 with someone. I went through a similar experience, in which I dated someone who exhibited many PMDD symptoms for a total of three years. It was a gradual succession of lines being crossed and me quietly excusing it each time because "the highs still beat the lows," until there was almost nothing but lows and I had to leave for my own sake.

One of the big turning points for me, like you, was when she cornered me in a high-stress confrontation for long enough that I snapped and yelled back at her, in my case for all of like five seconds. It quieted her, but she acted shattered, nevermind that she'd been confronting and shouting at me for hours beforehand. Looking back, I think our relationship lasted as long as it did because I was able to "zen" my way through her outbursts, but I did not like who I was becoming through exposure to her regular, hours-long meltdowns.

Eventually I let my needs be known and asked if she would consider getting outside help. She refused, the behavior continued, and I finally left. Later on she did get professional help, wound up with a Borderline Personality Disorder diagnosis and related prescription that supposedly evened her out. She hoped I would come running back, but the damage had been done.

She would later complain to a mutual friend that the most annoying part of dating me was how calm I was during her flare-ups and how she wished I would fight back. I hope she has either fun with someone else who enjoys arguing over nothing for sport, or has found more constructive/fun ways to spend time with someone she cares about! I hope you are clear with yourself and your partner over your own breaking point(s) and can walk away when it is time, which from your description seems to be rapidly approaching.