When do you know that your ready to have sex for the first time?

I've been dating this guy for around 4 months now, I'm 18 and i've never had penetrative sex with anyone before in my life. However, I've had my fair share of makeout sessions and groping. With this guy, I've been the most comfortable (Sexually at least) but whenever we're making out i feel like he wants to have sex with me, and since I'm turned on i let him do what he wants but we've never gotten around to doing it. I also don't think that my relationship with him will last very long because i (hate myself for it but can't help it) feel superior to him sometimes. I'm worried that i'll rush myself into it and i don't want to do it unless i know I'm ready. Any advice for holding myself back?