Neil Sedaka was right - breaking up is hard to do. Often it involves yelling and dividing up cds that haven't been played in a decade. Sometimes it involves one person being completely surprised. It always involves at least one person crying.
Why make it worse by being the ass who leaves without even saying it's over? If you're unsure what to say, try the old standby, "This isn't working" or even the cliched, "It's not you; it's me" - just say SOMETHING so the other person isn't left staring into a half-empty closet at 5:18 p.m. one Thursday.
A version of this is happening to one of my closest friends right now. She's not even sure if they've really broken up, things were so abrupt. One day she was helping her boyfriend's kids with their homework and the next - *poof* - he's gone.
This, of course, has me thinking about the big breakups of my own life: the ex-husband who answered, "Fine," when I told him we needed to divorce, the ex-boyfriend who managed to leave three nasty messages on my machine during my fifteen-minute commute from dropping off his things, and even the time my husband and I almost got divorced eight years ago (the roughest of all and we stayed together). Those events were tough, and I get why a person would want to avoid them, but come on! If you've ever loved a person at all, you owe them that final scene, however hard it may be.
Do not text and do not run in to let her know it's over while your new girl waits outside in a running car (which happened to a client of mine). Have the balls to say good-bye the right way. Expect tears, anger, yelling, resignation - expect it all. If you can, tell her something concrete, "I have feelings for someone else" or "I need someone who'll buy Charmin without bitching about the price." Be kind, too. You're the one leaving; you can afford to do it nicely. Say you're sorry, you wish things were different, but they're not, etc...
And if you decide five days later that breaking up was a mistake, be prepared to kiss some serious ass.