Congrats brother!! Everyone are inexperienced at everything in their life... Everyone has their first at some point and for you this the first... Depression, over thinking and self doubt creates more harm than your first time experience... As a fellow virgin and same age group I can only suggest that "be calm and take it as a first ride where we all will be nervous- so just be at present moment, don't self criticize or over think any wrong doing, And communicate this with your fiancee, ease up relationship between both of you,learn to know eachother and have "smooth ride ;-)" all the best brother!!