Leave her alone, break up with her and go experiment with your hormones with someone who is actually ready and willing to be with you sexually.
You're too young to be in a committed relationship with an 18 year old that is afraid of sex anyway. Wear a rubber every time you have sex and NEVER leave your chances of not getting someone pregnant up to the girl. Where a rubber until you are truly committed and are financial and emotionally mature enough to take care of yourself, your WIFE and your baby. That means wearing rubber even if she tells you she's on birth control until you have a full time career and a ring on her finger.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion