Talking to my mom about birth control

I have finally decided that if I'm ready to have sex I'm ready for birth control. I don't know how to talk to my mom about this. She is very over protective and has trouble letting me grow up. I am 18 and sexually active but I'm on my parents insurance so I am going to have to mention it to her. I cannot make up a lie about my period being heavy or painful because she already knows that's not the case. This is the main reason I have been putting off the idea of BC.