Is it normal to want sex but back out when the opportunity comes?

So im a girl, and i have a fairly active sex drive. Im a virgin, but i masturbate frequently. I experimented with another girl, and we did some stuff but broke up a while ago. I now have a boyfriend, and im sexuality attracted to him. I want to have sex, but when we're together and start getting physical i get nervous and stop.

I get scared about how ill be to him, and what to do, and if ill even like it. Is this normal?