When She Has Her Period
Your girl has a "grace period" for about a week every month in which you have to stick it out. Science is yet to prove this, but the emotional discomfort and physical pain endured by women during their periods gives them a superhuman ability to fuck shit up, so don't dump her during this time. Keep a calendar so you know her cycle and break up with her during one of the 3 weeks when she's not on her period—if you know what's good for you.
It's been dubbed "Shark Week" for a reason.