Four Daughters, Four Moms, Four Sex Talks

I'm 18, and I've been sexually active for about three years. I met my current boyfriend in August of 2010 and we've been inseparable since. He just celebrated his 21st birthday. My problem is, my mom seems to think I'm her angelic, virginal teenager. (I'm one of five kids) She doesn't know I'm dating or that I'm not a virgin. Before I go away to college, I'd like to come clean to her. I'm just not sure how to do that without shattering her image of me completely, though it seems inevitable.
 So, how do I begin to tell her?