There could be many causes, and this issue is complex with so many variables. I think there are unrealistic expectations on what sex is supposed to be. You are young and sex is a new experience for you and your body. Don't be too quick to attribute your lack of sex drive/enjoyment to coming to the conclusion that something is wrong with you. Take some time to get better acquainted with your own body. If you are not enjoying sex, you and your partner might want to take this as a sign to slow down. The act of sex is nuanced, as are your responses. I think you will come to find that sexuality and the desire to have sex will ebb and flow in your life. Become very familiar with your own anatomy. Touch yourself and look at yourself. Become comfortable when communicating your feelings with your partner. It is completely normal and okay to say no to your boyfriend. This will work itself out. Show yourself some grace, and take time to learn what you enjoy. An orgasm is not a measure of confidence. Your sexual desire is an exquisitely unique expression of individuality, and comparisons serve no one.
Maybe it's just your body telling you that you would rather not be having sex. Why do you do it, are you thinking you won't have a boyfriend if you don't sleep with him?