I totally agree with imkeen- if you've been with her since high school she might feel like she's "missing out" on "all the other expierences she could have had." I'm not saying that it's impossible to find true love before the age of oh, 21, but you do a LOT of changing between ages 14 and 22 or so- she could just be feeling wanderlust.
Let her go, tell her not to let the door hit her on the way out. Perhaps in some time, she will come back, perhaps not. I CAN tell you this from expierence that LIFE IS TOO SHORT TO BE WITH SOMEONE WHO MAKES YOU MISERABLE!!!! I was with someone for a year who made my life a living hell (didn't cheat on me, but plenty of emotional abuse, which is what you're in for if you stay with her- cheating is a form of emotional abuse). That was over two years ago, and I STILL have some residual issues I've not worked thru. Please, for your sake, give yourself some space to greive, grow, and get on with your fun life!!
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Sage knows our mythic history, King Arthur's and Sir Caradoc's
She answers hard acrostics, has a pretty taste for paradox
She quotes in elegiacs all the crimes of Heliogabalus
In conics she can floor peculiarities parabolous
-C'hi