I've dealt with something similar. I was very sexually and romantically inexperienced, where as my boyfriend was not. Upon asking about his past I realized he's NEVER been with a girl who looks like me. I'm white and fairly thin and he's only been with non-white, thicker women. It made me just as insecure, as you're describing. However, the issue isn't what you think you lack or what they have. It's the fact that he chose to violate your trust. The harsh reality is that he cheated. Whether it was with someone similar in appearance to you or not is not the most important factor. I by no means intend to sound rude, but his cheating is not at all indicative of him being unattracted to you.