don't know how to communicate with mother

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This is the hardest thing I've had to do yet. I'm 17 & have been with my boyfriend for 6 months. We've had sex multiple times and I've spent the night so often I lost count, my parents never knew. Until my mom found out this morning. She said she needed to talk to me and when we did it was absolutely pointless. She said that if I was not honest with her about spending the night then she would've taken my car. I told her the truth that I did stay the night, and she said I'm not allowed to be doing that stuff until I'm 18 (she doesn't know I had sex with him and still thinks I'm a virgin). This is the part that absolutely baffles me because what does my age have to do with anything? I graduated high school an entire year early, I have two jobs, a car, a license, and am starting college soon. Unfortunatly not in a dorm so I'll still be living at home. She couldn't come up with a reason to why 18 so she told me just to accept it because she's making it a rule. She said that if I start staying the night at his house then "stuff" will happen, and I asked her if that was seriously the worst thing in the world. She acts like my virginity is tied to my innocence and purity or something and it disgusts me. She said that I can not have sex until I'm married, and if I do I will get hurt. I told her that isn't true and she told me If I didn't stop arguing with her she would tell my dad what I did. I wasn't even arguing I was just trying to communicate with her. I want to be honest with her and tell her everything but she's like a brick wall and I fear any consequences. I started crying really bad after she said she was going to tell my dad and she literally just got up and walked away from me. I don't know where to go from here. I love my boyfriend so much and he lives 30 minutes away, if I'm not allowed to see him I'm going to lose my best friend and that hurts me so much. Please help and give me advice..