She is having fear of penetration. This fear is learned fear, she might have heard (e.g. from friend or colleague), seen (e.g. in movie etc.) or read (e.g. in book etc.) that it causes pain. You need to go slowly about it. First, you assure her that you are not going to penetrate without her permission and not going to cause any kind of pain to her. Engage in sexual activity without penetration (i.e. no intercourse). Once the comfort level increases try to go further, may be with finger initially and then penetrate. This may take 2 to 3 months also. If you not find this approach working then consult psychiatrist.