How to tell my mom

My mom has been dating this guy for almost a year. He was very nice at first and gave me presents and things. Eventually he started to get more closer to me and wanted to spend more time alone with me. He started to ask me things like if i’ve ever had sex and if I ever wanted to have sex. He comes in my room at night and caresses me and asks me to have sex with him. I was really nervous and didn’t know what to say. He started to touch me and told me that sex with him would be better because he’s experienced instead of anything with my boyfriend. I said no and he left my room because i threatened to scream at him. The next morning was very awkward and I didn’t want to talk to him. My mom got mad at me and I got pretty upset and I left the house. I went to my boyfriend’s house and stood there for a couple of hours until my mom came to get me. I feel so gross. I’m 15 and I’m scared to tell my mom because i don’t think she’ll believe me. I feel embarrassed to tell her.