Lets put it this way..if you didn't feel this way, you should be worried.
Its extremely nerve-wracking after graduation, where you have been guided through everything, to go to being an RN where your license is on the line.
However, give it some time. You will begin to feel more and more comfortable every day that you are there. You will feel more like a nurse as time goes on. You are still working with a preceptor, so use that to your advantage. Ask tons of questions, and learn many new things. Use what you have learned in nursing school and apply it to your new job.
It took me over a year before I felt confident, maybe a year and a half before I was comfortable calling/questioning Drs, and now at 2 years, I feel like a nurse. With that being said, I'm not completely confident in my abilities. I feel like I have so much more to learn, as will you, as well as other new graduates out there.
It does take time. I know its not that comforting to hear, but its true. You will get better and better every day. And dont fret about it. You're human, and you're a nurse. Also, by thinking and questioning everything, you'll be less prone to mistakes.
You'll do fine in your nursing career, and time will go by before you know it, and you'll become more and more confident. I promise
Best of luck