How can i tell my mom i am having sex?

I am 17 and i want to tell my mom i am having sex with my boyfriend. She already told me she doesn’t want me to have sex or do any sexual stuff with him bc she thinks i am really young and we have only been dating for about 5 months. She really likes him tough. I don’t know how to tell her… and I really want to, specially bc i want to get on the pill. But i am afraid i tell her and she will keep me from seeing him… How can i tell her that? (Just keep in mind she lost her virginity to my dad after their wedding).