Telling my mom I'm not a virgin.

So here's my dilemma. I haven't been a virgin for going on three years and I've kept that from my mom. I use the natural family planning birth control with basal body temperature reading every morning. I've been doing this since I lost my virginity. I'm 18 now and I still don't see why she needs to know. That and I'm just terrified to tell her. That's an awkward conversation. I've been thinking about this a lot because my boyfriend is urging me to tell her. Should I? And if I should how do I go about that?