Has she given you any indication that she's been unfaithful? If not, it's your insecurity and that will drive her away. Leave the past there. It's hard to, I know, I am struggling with the same issue. When you start thinking about her sexual life with this guy, stop yourself, take a deep breath, and breathe out until you can't feel your anxiety any more. Then take the images in your head, and crumple them up and put them in the trash. Now think of all of the fun sexy times you both have had. And stop...I know once you start doing this your will wonder how your sex compares to his. Just stop. Don't worry about it..if she didn't like it she wouldn't be with you. As a girl I can tell you, as long as you do what you're told and/or listen to our bodies...sex is good. Take care of yourself, work on the root of your self esteem and insecurity issues, and focus on the good.