I am glad you are taking thoughtful reflection on your behavior. Your concern about your capacity for intimacy is the essential ingredient in trying to unravel the dynamics and make changes. I recommend two things: First I would use the “find help” tab at the top of the page to locate a therapist in your area so you can begin this reflective journey with someone who has navigated with others. A good therapist will help you pose, and reflect on, the questions you have and help experiment with changes.
Second, the anxiety you feel is reflected in the partner you chose. We tend to be drawn to the people that resonate with our emotional status. For more information on this please check out part one and part two of these articles I have written on this phenomenon. What this means is that both you and your girlfiriend are likely to be struggling with issues surrounding intimacy. The way for you to evolve through this is to see if you can have a dialogue with her about your feelings and in this way perhaps the two of you can each help the other.