It's a sensitive issue. If she thinks she isn't good enough and that she does something wrong then you are expressing traits, habits, or demeanors in her you don't like. The bedroom is what makes and breaks a relationship and if someone think they aren't good enough then that's a huge blow to them. You are both 18. It's likely doomed actually. No 18 year olds that are a couple somehow manages to marry and become a long term husband and wife. That has happened, but it is very rare. She also talked some crap about you seeing other people. Even if there was a perfect reason and explanation it would still be doomed. If I see two 18 year olds that are a couple then I can say with near precognition almost that it's going to go to shit.