What strategies can I use to stop comparing myself to his ex?

I'm a 20-year-old woman struggling with self-confidence, and this weekend, I'm going to a small gathering for my boyfriend, Dan (21), and our friend, Mat, who is hosting a housewarming and Christmas party. Mat recently got engaged to his girlfriend, Sam, who he’s been with for nearly two years. Interestingly, Dan was in a situationship with Sam’s friend about a year ago. I often find it hard to feel good about my body and self-image. Although I’m fairly skinny at 5'6" and 130 lbs, I can’t shake the feeling that I have some “pudge” around my tummy because it isn’t toned. I also have what seems like a small butt, frequent acne, and not many friends. This weekend's party will have fewer than 17 guests, and Sam’s friend will be there too. Dan was uncertain about attending because of this history, but ultimately, he decided that supporting his friend was more important—especially since we’ll all be attending their wedding in less than a year. I'm feeling conflicted about how to handle this situation. I don’t want my insecurities to overshadow the event, but I worry that I won't be able to stop myself from comparing myself to her. If I choose not to go, I fear my imagination might run wild, and that could be even worse. I hate feeling so self-conscious, particularly about his past relationships. Dan has been with several people before, while I haven't dated since I was 15, and he's the only one I've ever been intimate with. He treats me incredibly well and is genuinely the best, but my anxiety and low self-esteem make me fixate on my perceived flaws. I often wonder if his exes were better in bed or more attractive. I'm at a loss about how to overcome these feelings. I’m tired of being hard on myself, yet I feel stuck in this mindset.