birth control?

I'm 16, 17 in less than 3 months. My mom knows I'm not a virgin but she doesn't want me to have sex. Being a teenager w hormones, I get horny. Even she's told me that once you do it, you want to continue to do it /: I always use protection but I want a backup in case anything goes wrong. However, I don't want to ask my mom because I know she won't be up for it & she'll think I'm having sex 24/7. Is there any way I can get on bc without my mom knowing? I have a cousin who once offered to get me on but she's the type that will tell her mom or her sister, then they tell my mom, then my mom confronts me. It's not the first time it's happened