My fear of sex

I don't really know how to say it, but I'm afraid of bottoming. I'm afraid that an. Accident. May happen. I've topped one guy before and I liked it, but I don't see myself as a top. I've hooked up with guys before. But never gone to sex before. I'm almost 20 and want to expand my sex life. But I'm just so afraid of the worst. I want to bottom. But my anxiety and fear stop me. Some advise?