If she was forced by a man to perform oral sex when she was younger, it would be wise for her to see a counselor for a few visits to deal with the experience. See Sexuality Resources for more information on sex therapists and hotline numbers. You can also remind her gently that you are a different person than the one who forced her, and that you would never do that to her. If it's about taboos against oral sex, it falls on you to debunk the myths she's living with. It will take some serious discussion to break down any of her stereotypes that have been built up over time.
Many women feel out of control when giving head.
Have her try using her hand to guide your penis into her mouth. That way, she can be in control of when, how far, and how fast your penis will move. You will have to experiment together to see what works for you. Most importantly, you'll have to be willing to forego the act at times if your girlfriend is really uncomfortable. Click on her picture for her bio.