Jetée

This situation's dramatics, originally written and starring WesMantooth's girlfriend, could have been set in motion by a number of things:
maybe she thinks you two are already 'serious', unbeknownst to you, and whatever signals she's picked up from you thus far; as chauvinist as it may seem of me, maybe she was punching her one 'on my period / hormonal rage' ticket every fertile/pregant/hot flashing woman gets each month or so, and she explains that as the reason for knocking over tables and crushing Japanese civilians; also, she may received some devastating new earlier in the day, like perhaps her pet cat got caught in the dish disposal and then she hit 'on', or she now has a creepy intern that stares at for hours on end at work, and finally comes "home" to her boyfriend, but sees you'd rather spend the night playing rock band with friends (I'm still on your side, though - these are just hypothetical explanations I'm offering here).


Best advice:
be upfront with her this weekend (especially now that I remember that America's-commercialized-day-of-investment-time-stamp, Valentine's Day, is tommorow) and ask her what the real deal was about the other night. Do you act that way around family? Once again explain your prior commitments in which you made and felt obliged to acomplish, especially to someone who would question your motives about it, and make her understand that such dramatics over trivially, nothing, will no longer be tolerated on your part. It's somewhat like dog-training in that if you don't set the ground rules, and don'ts, it will keep happening over and over again. I'm not sure how you would feel about being remembered (or mourned, worst-case scenario) as the guy who dumped the girl on V-Day, but let's just see what happens if you successfully make-up your affair; maybe you'll acquire an incredible night of passion! (or maybe just another day in which you didn't feel pressured into dropping multiple bills for chocolate and flowers, as mandated by the school system / society at large).

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As human beings, our greatness lies not so much in being able to remake the world (that is the myth of the Atomic Age) as in being able to remake ourselves.

—Mohandas K. Gandhi