The bottom line is that it's not fair to her to invade her private past , and ultimately, does not serve You or the relationship. Try to replace your obsessive thoughts with more positive ones about the happy state of your life and future with your GF. Fantasize about your current sex life. Talk to her for reassurance if you need it but don't pester. Consider yourself lucky to have found someone in this day and age with so few men in her past. It's rare.