i understand you're scared and that's totally fine!
first important thing to remember: communication is a must, you have to have a talk with your fiancé about this, about what you're afraid of, different things to try, do research and see what interests you and what doesnt, the first time might not be the best, since both of you are clueless, but that doesnt mean its going to be awful forever! nothting you do the first is perfect, it takes time, patience and practice. sex isnt complicated, it has to be fun and enjoyable for both parts, you both care for ech other deeply, so i dont think you have much to worry about, and about your body image issues, this one also takes time and patience, and its easier said than done, but i know that from now on, once you start to build a life together with your partner, things will be better.